Across:
8 Elaborate style (ROCOCO) of writing (from the 3Rs = R-) from two different officers (Officer Commanding + Commanding Officer = OC + CO = -OC-CO) divided by order (-O-) (ROCOCO),
9 Former (EX-) marine (marine), jockey (= anagram indicator) (marine = -AMINER) (EXAMINER) or judge (EXAMINER),
10 Recalled (= reverse indicator) Degas (Degas), for the most part (s) (‘Degas’ without ‘s’ = dega = AGED), getting on in years (AGED),
11 Make weak (DEBILITATE) argument (DEB-ATE) over leaders increasingly living in terror (leaders ‘Increasingly Living In Terror’ = -ILIT-) (DEBILITATE),
12 Loud noise (BANG-) from university (-U-) and institute (-I) (BANGUI) in African capital (capital of the Central African Republic = BANGUI),
14 Box (SPAR-) with brie (brie) crackers (= anagram indicator) (brie = -E RIB) (SPARE RIB) – was that pulled out for the First Lady? (SPARE RIB)
15 Etis oppoehtt suj (etis oppoehtt suj)? On reflection (= reverse indicator) (etis oppoehtt suj = JUST THE OPPOSITE), you couldn’t be more wrong! (JUST THE OPPOSITE),
18 It’s not healthy (SICK-) for the plants (-BEDS) (SICKBEDS) or for those looking poorly in the nursery (SICKBEDS),
20 Notice the absence (MISS-) of academy (-A-), league (-L) (MISSAL) and church publication (MISSAL),
22 Sparks problem (POWER SURGE is a spark’s problem/ electrician’s problem) for major nation’s (major nations = POWER S-) press (-URGE) (POWER SURGE),
24 School group (school of FISH) at the end of 3 down (at the end of ‘cold fish’ = FISH),
25 Father (REVEREND) English (-E-), at the end of summer (at the end of ‘summeR’ = -R-), goes into Denver (Denver) Order (= anagram indicator) (Denver = REV-END) (REVEREND),
26 Open letter dismisses plot (‘open letter’ dismisses ‘plot’ = eneter = ENTRÉE), of course (ENTRÉE).
Down:
1 Gallery in the house (LOGGIA) locked up (= reverse indicator) by Padraig Goldsmith (locked up by 'padrAIG GOLdsmith' = aiggol = LOGGIA),
2 Common complaint (COLD) of half of those in 3 down (half of ‘cold fish’ = COLD),
3 Unemotional type (COLD FISH) is familiar with the seas around The Artic presumably (COLD FISH),
4 Some sombre hotel set up (= reverse indicator) (some ‘somBRE Hotel’ = breh) (breh = HERB) by Basil (HERB),
5 Relative (Pa = PA-) met the Spanish (‘the’ Spanish = -EL-) and the French (‘the’ French = LA) (PAELLA) for dinner in Madrid (PAELLA),
6 Ducks (MIS-SES) found around most of (s) the lock (tress) (most of the ‘tress’ = -TRES-) (MISTRESSES) by the other women (MISTRESSES),
7 Car (model -T-) taken by teacher (Sir = -RIS-) up (= reversal indicator) in The Capital? (capital/money = CEN-T) (CENTRIST) The one found in the middle of the road! (CENTRIST),
13 Presumably, nothing gets by this person (GATEKEEPER) responsible for the playacting (GATE- Theatre) at The Guardian (-KEEPER) (GATEKEEPER),
14 Studies (SWOTS) all around (= reverse all indicator) heads of steeplechasing thoroughbreds (heads of ‘Steeplechasing Thoroughbreds’ = st = -TS) and farm animals (cows) starting off (c) (‘cows’ without ‘c’ = ows = SWO-) (SWOTS),
16 Needs to be pressed (UNIRONED) by a foreign (a = UN-) journalist (-ED) following (= positional indicator) club (golf -IRON- club) (UNIRONED),
17 Spoilt (PAMPERED) by spinning (= reverse all indicator) the German (the = der = -RED) record (Extended Play record = EP = -PE-) – a favourite of The Tourists (map = PAM-) (PAMPERED),
19 Oriental (East = E-) flower (-ASTER) (EASTER) is a spring annual that follows some hard and fast rules (EASTER),
21 Getting Noel out of New Orleans (‘Noel’ out of ‘New Orleans’ = newras = ANSWER) for the comeback (ANSWER),
23 Bars (RODS) for The Fringe’s rock bands (for the fringe’s ‘ROck banDS’ = RODS),
24 Footsteps in the snow leads (‘Footsteps In The Snow’ leads = FITS) to explosions, in a comical sense (FITS of laughter).