4020 Raj by Llig

This was a fairly straightforward puzzle from Llig. The extra letters in wordplay spelled out a quotation from the prologue to Shakespeare’s Romeo and Juliet.
“A PAIR OF STAR CROSS’D LOVERS” In the completed grid could be seen diagonally the names ROMEO and JULIET. These had to be highlighted along with the “stars” that crossed them at 180º, CASTOR and COMET. Gregson was for a while bemused by the Celtic (cross) that appeared in the grid. The title was simply the initial letters of the play.

Extra letters in wordplay are shown in []
 
ACROSS
1 John's bowled over by rock music's rhythm (4)
TALA LAT reversed plus A[A]
5 Get used to year's compliance with tax (8)
ACCUSTOM A (anno = year) + C (compliance) + CUSTOM
10 Intend spending recklessly, losing last of inhibition (6)
DESIGN Anagram S[P]ENDI(N)G less N
11 Twiggy knitted products for 'Arriet's underwear (6)
OSIERY  (h)OSIERY
13 A tiara's broken, losing a quarter at Scone (4)
AIRT Anagram A TIAR[A] less A
14 One to rush around start of hunt - it follows the scent (5)
RACHE [I] (one) H (unt) in RACE
16 Bling bling set round with borders of ruby (7)
JEWELRY W (with) in JEEL (set) + R(ub)Y
17 Taking cue somehow from Herculean chopping as Hydra's head is removed - here (5)
LERNA Anagram HERCULEAN less letters of CUE and H
19 Throw a turn and pack for absence without leave (7)
TRUANCY Anagram A TURN + C[R]Y
21 Could be Welsh rugby's leader goes for it in scrumming circle (6)
CELTIC Anagram CIRCLE swapping R for T (it)
22 Transformer's in contact with current in metal bar (6)
TOROID TO (in contact with) + I (current) in ROD
25 Excruciating pain in bone is followed by MS - mustard plasters required (9)
SINAPISMS Anagram PAIN in [O]S (bone) + IS + MS
28 Attendants look over one on board (6)
SAICES SS (on board) containing I in [F]ACE
30 Metric marks mostly direct me to indicated air speed (6)
SEMEIA SE(t) + ME + IA[S] (indicated air speed)
31 Pre-record farewell aria (7)
CANTATA CAN (pre-record) + TA-TA (farewell)
34 Athlete suffering setback in preliminaries (5)
MILER Hidden reversed
36 Valuable instrument had broken - it's offered to departed spirits (7)
SRADDHA S[T]RAD + anagram HAD
37 Industrious worker I objectively turned and opposed (5)
EMMET ME reversed + MET
38 First-class guide, a role model (4)
ICON [A]1 + CON
39 To set things alight, King leaves retiring in turmoil (6)
IGNITE Anagram [R]ETIRING less R (king)
40 Lease is inherited, subsuming freehold possession (6)
SEISIN Hidden
41 Intestinal trouble could end one (8)
DUODENAL Anagram [C]OULD ONE + A (one)
42 Bay perhaps can be found parking in street (4)
TREE Hidden
 
DOWN
1 Ferries cart jets struggling over river (8)
TRAJECTS R (river) in Anagram CA[R]T JETS
2 One has to depart with respectful title - goodbye (5)
ADIEU A (one) + DIE + U (respectful title in Burma)
3 Star no longer acting with alcohol, mostly (5)
ASTER A (acting) + STER[O](l)
4 Joint is mostly betel added to a bit of lime and half heroin (7)
SIRLOIN SIR(i) (betel) + L(ime) + (her)OIN
5 Variety of fungus might yield a cigar (6)
AGARIC Anagram A CIGAR
6 Company lacking part of a legion (6)
COHORT CO (company) + [S]HORT
7 Sword for example, Sergeant-Major holds one highly valued (7)
SIDEARM
8 Taser breaks up act of violence (4)
TEAR Anagram TA[S]ER
9 Link my DNA for exotic vertebrate with facility for speech (4)
MYNA Hidden MY [D]NA
12 Rotten leader fled first (6)
RANCID CID (leader) following RAN
15 Life force, with added energy, may stop cell recombining (9)
CYTOPLASM Anagram MAY STOP CEL[L] less E energy
18 Scots bore drivels interminably enthralling one (5)
ELSIN Hidden
20 Snooper losing power and beaten about, remains a snooper (5)
NOSER Anagram SN[O]OPER less P (power)
23 Sparkling stuff - man's taken by one served during engagement (8)
DIAMANTE I in DATE containing MAN
24 Drink in overindulgence almost, crying like a lamb (6)
BAAING A[V]A in BING(e)
26 American singer excitedly recited opening of Iliad (7)
ICTERID Anagram R[E]CITED + I (liad)
27 Describing bird of paradise perhaps, thus overwhelming frenzied mate (7)
SEMATIC Anagram MATE in SIC (thus)
29 Ants stagger over me, a pollen producer (6)
STAMEN Anagram ANTS containing ME
30 Staying power protects bishop to wander in Ireland (6)
STREEL R[R] in STEEL
32 Poet entertained by Hood is tragedian (5)
ODIST Hidden
33 Scottish lament, something expressing sorrow by measure of sound (5)
OHONE OH! + [S]ONE
34 Young girl's crazy about one (4)
MAID I in MAD
35 Fabric contributing to woollen overalls (4)
LENO Hidden