3961: Invitation by Duck
 
In his Christmas puzzle, Duck gave the solvers enough information to complete the theme. The initial letters of extra words gave the message ONE LETTER GAP IS CROTCHET – DRAW STEMMED NOTES. This referred to the clashing letters and implied that two letter gaps were minims. Extra letters in the wordplay of seven answers gave the letters of notes applied to each line. Thus the solver could recognise the tune of Adeste Fideles and enter stemmed notes for the clashes and write the first 2 lines of the song (O come all you faithful, Joyful and triumphant) in the space beneath the grid.

Extra words in clues and extra letters in wordplay are shown in square brackets.

ACROSS
1 Annoy Henry, a right [old] fool (6)
HARASS H + A R + ASS
6 [Naval] warrants East Caribbean guard put aboard ship (6)
SEPADS E[C] + PAD in SS
10 Weary? Be engaged in [endless] rows (6)
TIERED [B]E in TIRED
11 Understanding school association will provide input for instrument [lessons] (6)
UPTAKE PTA in UKE
14 Go before President, go back [empty-handed] (7)
PRECEDE P (president) + RECEDE
16 [Teenagers] waving hairbands around (8)
BRANDISH anag. H[A]IRBANDS
17 Birds crossing river step [tentatively] as before (6)
GREESE R (river) in GEESE
19 [Environmentalist's] against India's native mink (5)
VISON V + I + SON
20 Rock star meeting [relations]? That may be in 27 (10)
MARTENSITE anag. STAR MEETIN[G] may be in STEEL (27)
22 Old [geezer] brought in a bit of money - one joining concert audience (10)
STAGE-DIVER AGED (old) in STIVER
27 Good person [admiring] slippery person's nerve (5)
STEEL ST (saint=good person) + EEL
28 [Plucky] soldiers confront threat (6)
MENACE MEN + [F]ACE
30 Ban a cure that's involved a wax [implant] (8)
CARNAUBA A in anag. BAN A CUR[E]
33 Philosopher prepared article admitting [stupid] blunder (7)
DERRIDA ERR in DID A
34 Sea creature misplaced in [chestnut] tree (6)
NERITE anag. IN TREE
35 Frenzied female forced [randy] man back (6)
MAENAD MA[D]E + DAN reversed
36 Small Scottish vessel [off] Australian island (6)
TASSIE 2 definitions
37 Introduce knight in weird [talismanic] rites (6)
INSERT N (knight) in anag. RITES
DOWN
2 Alternative to tumble-dryer superior - no loud [clanking] (5)
AIRER FAIRER less F (loud)
3 Direct attention to nab Ecstasy, [heroin], form of cannabis (6)
REEFER E in REFER
4 [Expensive] bridges - profligate research engineers sacked (6)
ARCHES anag. RESEARCH less RE (Royal engineers)
5 Saw illiterate [town] children (4)
SEED 2 definitions
6 [Drudge] cuts sticks, we hear (6)
STEAKS Homophone STAKES
7 Joke poem once [riled] expert (6)
PUNDIT PUN + DIT
8 Port makes singer lose temper [alas] (4)
BARI BARITONE less TONE
9 [Who's] getting cross, fantastically energised? (9, two words)
SEEING RED anag. ENERGISED
12 If running short, supply [supplements] (7)
PROVIDE PROVIDE(D)
13 What I find painful? Indian city [transport] (6)
MYSORE My sore!
15 Like a domestic match with nice [English] gents playing (9)
IGNESCENT anag. NICE GENTS
18 [Muddy] water, stale water? This should keep out any water (7)
SEALANT SEA + LANT (stagnant water)
21 Enzyme could make cat [miaow], yes? (6)
CYTASE anag. CAT YES
23 Braid put up for game [embellishment] (6)
TENNIS SINNET reversed
24 Gee - little good in [dugite's] violent venom! (6)
GRUDGE G (gee) + G (good) in RUDE
25 Artist occupying imposing [Norman] residence (6)
INGRES Hidden
26 Polynesian wife's [overpowering] greeting when coming within sight (6)
VAHINE HI in VANE
29 Tree [trunk] man placed in vehicle (5)
CEDAR ED in CAR
31 Possibly Muscovite tricks to expel English [envoys] (4)
RUSS RUSES less E (English)
32 Horse, not a second-class one like a [steeplechase] jumper (4)
ARAN ARAB less A B + AN Ref. Aran sweater