Across:
8 Spoilt (spoilt) soup (= anagram indicator) (spoilt = PISTOL) as a starter and was fired? (starting PISTOL),
9 Sowing the seeds (STARTING) of agony (sting = ST-ING) over a (-A-) retweet (-RT-) (STARTING),
10 Underhand type (PALM) of participating Stockholm banks (‘PArticipating stockhoLM’ banks = PALM),
11 Nominated to an office (DESIGNATED) as agents died (agents died) all over the place (= anagram indicator) (agents died = DESIGNATED) (DESIGNATED),
12 Unties (unties/frees = LETS GO) the French (‘the’ French = LE-) swimming (= anagram indicator) togs (togs = -TS GO) (LETS GO),
14 Get it in the neck from the butcher (SCRAG END), barman (Senior Counsel = SC-), artist (Royal Academician = -RA-) and legend (‘leG’ end = -G) leading to wind-up (END) (SCRAG END),
15 They get things going with a bang for those on the run (STARTING PISTOLS) in 9 across (‘starting’ = STARTING) with more than one in 8 across (more than one ‘pistol’ = PISTOLS) (STARTING PISTOLS),
18 Finish off (e) light fluffy dessert (‘mousse’ without ‘e’ = MOUSS-) also known as (Also Known As = -AKA) (MOUSSAKA) a Greek favourite (MOUSSAKA),
20 Rotating (= reversal indicator) switch (on = -NO-) in Cork (CA-P-) yard (-Y) (CANOPY) provides shelter from the sun (CANOPY)
22 Provides a warning to those travelling overseas (LIGHTHOUSE) with match (LIGHT-) crowd (-HOUSE) (LIGHTHOUSE),
24 Open 13 down (open ‘SURF school’ = SURF) to ride a white horse? (SURF),
25 Water (RESOURCE) facility? (RESOURCE),
26 Where you’re bound to learn a lesson (SCHOOL) at the end of 13 down (end of ‘surf SCHOOL’ = SCHOOL).
Down:
1 Turns up (= reversal indicator), for example (e.g. = -GE), with a (-A-) short (t) inventory ('list' without 't'= lis = SIL-) (SILAGE) of fodder (SILAGE)
2 A little bit (ATOM) of what’s served up (= reversal indicator) in Palermo takeaway (in ‘palerMO TAkeaway’ = mota = ATOM),
3 Tips for grilling langoustines dipped (tips for ‘Grilling Langoustines Dipped’ = GL-D-) in some garlic mayonnaise (aioli = -A-ILOI) (GLADIOLI) for relatives of Iris (GLADIOLI),
4 Snakes (ASPS) and newts removed from wasp’s nest (‘newts’ removed from ‘wasp’s nest’ = asps = ASPS),
5 Harry (harry/pester = BADGER) has awful (BAD-) head gash (head ‘Gash’ = G-) and is in hospital (-ER) (BADGER).
6 Great hints (great hints) – wrong (= anagram indicator) (great hints = STRAIGHTEN) or correct? (STRAIGHTEN),
7 Recruit (INTERN-) a (-A-) novice (learner = -L) (INTERNAL) in-house (INTERNAL),
13 Teaches students how to go from site to site (SURF SCHOOL) by instructing classy types to go on the board, in the main (“the main” = open ocean) (SURF SCHOOL),
14 Greek character (18th character of Greek Alphabet – SIGMA) employed by a general employer (GI = -IG-) retires (= reverse indicator) close to Potchefstroom (close to ‘potchefstrooM’ = -M-) in South Africa (SA = S-A) (SIGMA),
16 Brief (remit = - TIMER) women (-W-) consumed by good books (Old Testament = OT = T-O) all about (= reverse indicator) (TWO-TIMER) unfaithful lover (TWO-TIMER),
17 Become greater (INCREASE) with a sincere (a sincere) shift (= anagram indicator) (a sincere = INCREASE),
19 Jockey (= anagram indicator) a horse (a horse = ASHORE) on dry land (ASHORE),
21 Overlook (overlook/forgive = PARDON) short (t) role played (part) (‘part’ without ‘t’ = PAR-) in the mob (-DON) (PARDON),
23 Characters involved in refuse disposal (characters involved in ‘refUSE Disposal’ = USED) or second-hand stuff (USED),
24 Starts screaming one’s head off (starts ‘Screaming Ones Head Off’ = SOHO) in London (SOHO).