4079 Sine Qua Non by Shackleton

Shackleton gave us a veritable tour de force with this elegant multi-layered puzzle. The corrections to misprints spelled – “Dot one’s Is and cross one’s Ts” and “Iddy Umpty”. This was a cryptic instruction to treat the letter I as a dot and the T as a dash in the normal clues. Using Morse code the solver was able to find “Must it be?” and “Muss es sein?”, which Beethoven had written in his manuscript of his Quartet in F Major Opus 135. The penultimate element of the first quotation was the letter B (-•••) and the only other letters that are made up of one dash and 3 dots are L, V and F. This was confirmed by the answer PAEON, a foot of 4 syllables, any one long and three short. These letters had to be highlighted, giving the initials of the questioner, Ludwig van Beethoven and the key of the piece, F, as well as the word MAJOR.
Treating the letter V as the number 5, solvers had to check the 5th letter in the 5th word in the normal clues. This gave “Diametric exchanges”. Swapping the letters of PAEON with their diametric opposites in the grid gave the message “DIT DIT DIT DAH”, the Morse code for V and the opening notes of Beethoven’s Fifth Symphony. To make the grid conform with this piece the final touch was to alter the highlighted key from F major to C MINOR!
The title referred to Beethoven’s answer to his question – “It must be!”

Corrections to misprints are shown in [ ].

1 Bridge partners ask about bet - place tenner, vulnerable? (8, two words)
WEAKSPOT [tenDer] W + E (bridge partners) + Anagram ASK + POT
8 Plant with yellow flowers found by Dutch caver (5)
DRAPE [cOver] D + RAPE
13 Hovels not initially covered by insurance (4)
INNS [hoTels] N(ot) in INS.
14 With return of diabolic black beast, it might help to examine myth? (7)
LIVE-BOX [mOth] EVIL reversed + B + OX
15 Old representation of map? It contains intricate design (6)
STATUA [maN] TATU in SA (sex-appeal = IT)
16 Cool till temperature's 200 and put in gravy (7)
INEARTH [gravE] IN (cool) + EAR (to till) + T + H (200)
19 Perhaps referring to stroke in eighty year old, retired - ultimately monoplegia's assumed (4)
OARY [eightS] (monplegi)A in YR + O reversed
20 Deposited ice chunk (4)
RIME [chInk] 2 definitions
22 Roughly eleven burhels in river (5)
HOMER [buShels] 2 definitions
23 Fine individual ingredients of a bottle for Edmund? (4)
FONE [bAttle] F (fine) + one
24 Mend the spool-shaped instrument (5)
CURET [spooN-shaped) CURE + T’
26 Green bird's last pair seen in diminishing habitat - vireo? (13, two words)
31 Pile of wool, Scottish wool, flapped out (5)
NAPOO [Clapped out] NAP + OO
34 Pout - married with little support (4)
TEEM [pouR] TEE + M (married)
36 Gritty silica removed from around stackyard (5)
TACKY [grOtty] sTACKYard less SARD
38 Ely cathedral is in need of repair - odd details withheld (4)
ARCH [Sly] Compound Anagram CATHEDRAL IS less anagram DETAILS
39 At start of evening would cook underground item from plant (4)
EDDO [Stem from plant] E(vening) + ‘D (would) + DO
43 Leaders of race take enjoyment in failure - not a chicane! (7)
NORTEÑO [not a chicanO] Initial letters R(ace) T(ake) E(njoyment) in NO-NO
44 Around city, working, it's very mad in the street (6)
ANYONE [maN in the street] NY + ON in AE
45 Though not fully developed, loaf-cutter possibly delivered greatest cut (7)
ERGATES [lEaf-cutter] Anagram GREATES(t)
46 Pretence one found in king and queen? (4)
MIEN [preSence] I in MEN (chess pieces)
47 Vase of ancient lime beginning to decompose a little bit (5)
DIOTA [vase of ancient Time] D(ecompose) + IOTA
48 Worried about becoming tarty? (8)
DESSERTS [tartS] STRESSED reversed
1 Believe '60's teenager about quality of mags (6)
WISDOM [of magI] WIS + MOD reversed
2 Rum in tea for boss? (5)
ENTIA [boDs] Anagram In TEA
3 A bit of nerve degeneration hospital's ignored - reversal in future seen? (8)
ANATROPY [in future seeD] A + N(erve) + ATROPHY less H (hospital)
4 Without question, Scots savour English fogs (6)
SQUARE [fogY] QU in SAR + E
5 Sunless spell - one that's cloudy and dark (6)
PHAEIC PHASE less S + I _ C (cloudy)
6 Mark leaves, loosely wrapped, flaying female variety show (4)
7 Can packer having meagre anorak finish early? (6)
9 Make sense of short dream, surreal (4)
READ Anagram DREA(m)
10 Reddish brown? Esau's granddad expresses cry of surprise (5)
11 Unlawful booze packaged by depot he enlarged (7)
12 Executive runs when impeding powerful urge (6)
EXHORT EX  + R (runs) in HOT
17 Heavens above! Spenser's easy reading when severely pruned (5)
ETHER ETHE + R(eading)
18 Snappy new clothes hanger? (4)
PYNE [hUnger] Hidden
21 Could show Suffolk girl around capital (5)
MUCOR [Mould] UC (upper-case) in MOR
25 Merrier drinker seen in Algiers? Spotted one in north (8, two words)
PIED NOIR [Perrier drinker] PIED + I in NOR
27 A judge separating native from greaser (7, two words)
A MAJORI [greaTer] A + J (judge) in MAORI
28 Yankee hosted by queen - Earl shed some tears for her (5)
RAYNE [Years] Y in RANEE less E (earl)
29 Act of French ... of French taken aback (4)
DEED DE (of in French) + DE reversed
30 Removed pips from fruit, canned (6)
STONED 2 definitions
32 Variables aside, amended pay index caused unease (6)
PAINED Anagram PAY INDEX less X + Y
33 Group in deep space unlocking energy from hydrocarbon isomers (6)
34 Judges spare an OK neighbour (6)
THINKS THIN + KS (Kansas! Neighbour of Oklahoma OK)
35 Masses had lowly seaman embracing religion (6)
DOZENS ZEN in ‘D (had) + OS (ordinary seaman)
37 Load minicab perhaps in advance of journey (5)
40 Duke's nearly drunk, and using French grammar (5)
41 Rank English bric-a-brac (nearly valueless) (4)
42 Doctor does give some drugs (4)