Solution to 12A has been deliberately left unsolved and is to be answered by a non-regular/novice commenter, with proper annotation. Those who have answered earlier in the week, please give others a chance.
Open for anyone to answer, if not solved by 1 PM.
ACROSS
1 Special ceremony for fairy (6) SPRITE {SP}{RITE}
4 Documents show public school circumventing copyright? (6) PAPERS {P}{APE}{R}{S}
8 Sear easily on burning range (7) EARSHOT {SEAR}*{HOT}
9 They're gutted when taken in by nurse provider (7) CATERER {CA{Th..rE}RER}
11 Try putting it back on head for warmth (10) HEARTINESS {HEAR}{IT<=}{NESS}
12 Card so far held back shows clover, in the end (4) T?E? (Addendum - TREY {YE{c...eR}T<=} - See comments)
13 Often suppressing secret need for drama (5) SCENE {SeCrEt+NeEd}
14 North African King, starting to convulse, is returned to infirmary (8) SICKROOM {MOOR}{K}{Co...e}{IS}<=
18 Antarctica's western end: area surrounded by water, you could walk down one (5) AISLE {a...cA}{ISLE}
20 Green part of body that's got teeth (4) GEAR {G}{EAR}
21 Make defensive walls of credit European banks and the Spanish get behind (10) CRENELLATE {CR}{Eu...aN}{EL}{LATE}
23 Injured in the back, hobbling around intensive care, returning often? (7) DECIMAL {LAM{IC}ED}<=
24 One that argues to forbid boxing then intermittently leaving (7) DEBATER {DEBA{ThEn}R}
25 Tension as IT tried regularly to get SAAS contents released (6) STRESS {aS+iT+tRiEd}{SaaS}
26 Avenues not very new, causing anxiety (6) UNEASE AvEnUES*
DOWN
2 Book on verse's translated (7) RESERVE {RE}{VERSE*}
3 Rats without front teeth in shed shooting up empty rafters (3-6) TWO-TIMERS {WO}{Te..h} in {EMIT<=}{Ra...rS}
5 Group from North-Eastern state cycling (5) AMASS (+am)AMASS(-am)
6 Citizen peeled carrot, celery on the counter (7) ELECTOR [T<=]
7 Dock worker's girl walked in, strode out (9) STEVEDORE {EVE} in {STRODE}*
10 Perhaps when hot husband's out traveling, it can become a daily issue (9) NEWSPAPER {PERhAPS+WhEN<=}
13 Quiet amateur at gym - smaller and lacking form (9) SHAPELESS {SH}{A}{PE}{LESS}
19 Noblewoman from America's uppity articles about Lithuania (7) SULTANA {US<=}{AN}{A} over {LT}
21 Boorish and angry orator's starting to be kicked out by head of administration (5) CRASS CR(-o+a)ASS
22 Setter supports article's topic (5) THEME {ME}<=>{THE}
Reference List
Special = SP, Public = P, School = S, Copy = APE, Right = R, King = K, Green = G, Credit = CR, The in Spanish = EL, Intensive care = IC, Very = V, New = N, Hard= H, Time = T, On = RE, Without = WO, Girl = EVE, Hot = H, Husband = H, Quiet = SH