Across:
8 Gulf (gulf/rift = BREACH) War ending ('waR' ending = -R-) in seaside setting (B-EACH) (BREACH),
9 Swore (PROMISED) to delete (-D) text (prose = PRO-SE-) about Frenchman (-M-) on island (-I-) (PROMISED),
10 Extremes of Security Council (extremes of ‘SEcurity councIL’ = seil) bringing up (= reverse indicator) (seil = LIES) statements that aren’t true (LIES),
11 It’s hard to miss (EYE-CATCHER) checkout (check out = EYE-) for fish (-CATCH-) and a fair amount of (half of) beer (half of ‘beER’ = -ER) (EYE-CATCHER),
12 Is able to (CAN-) get this (this) mostly (s) (‘this’ without ‘s’ = -THI) (CANTHI) from the corner of one’s eyes (CANTHI in the corner of eyes),
14 Local wheeler-dealer (OPERATOR) or a trooper (a trooper) on the rampage (= anagram indicator) (a trooper = OPERATOR),
15 8 across (‘breach’ = BREACH) has nothing (O-) to do with note (music note -F) from 9 across (promised)? Brief (d) (‘promised’ without ‘d’ = -PROMISE) (BREACH OF PROMISE) in case that ends up in court! (in case that ends up in court = BREACH OF PROMISE),
18 Dealer in the docks supplies (dealer in the dock’s/ships’ supplies = CHANDLER) cocaine (C-) to spy’s boss (-HANDLER) (CHANDLER),
20 Lose track of (FORGET) furnace (FORGE-) temperature (-T) (FORGET),
22 It’s apparent for all (DETECTABLE) to see (DETECT-) on a (-A-) lawyer (Barrister-at-Law = -BL-) from Spain (-E) (DETECTABLE),
24 Current (FLOW) followers dismiss loser (‘followers’ dismiss ‘loser’ = folw = FLOW),
25 The Queen (BED-), The Office (-ROOM-) and End of Days (end of ‘dayS’ = -S) (BEDROOMS) –– presumably, the insomniacs never tire of them? (BEDROOMS),
26 In the market (TRADER) for square (T- square) pirate ship (raider) with no third sail (third ‘saIl’ = i) (‘raIder’ without ‘i’ = -RADER) (TRADER).
Down:
1 Middle Eastern (middle 'easTern' = T-) Republic (Republic of Ireland = -ROI-) gets kick-start ('Kick' start = -K-) from a (-A) (TROIKA) group of financiers (TROIKA),
2 Limits cabinet ministers (limits/sides ‘CAbinet ministeRS’ = CARS) having more than one for the road (CARS),
3 Embarrassed (SHEEPISH) by detective (-PI-) in Hesse (Hesse) novel (= anagram indicator) (Hesse = SHEE-S-) getting high (-H) (SHEEPISH),
4 Apostle lost lot (‘apostle’ lost ‘lot’ = apse = APSE) in the section of the church (APSE),
5 In retrospect (= reverse indicator), for example (e.g. = -GE), even Ian Thorpe (even letters of ‘iAn ThOrPe = atop = POTA-) (POTAGE) in the soup (POTAGE),
6 Repudiation (DISCLAIMER) of Dee (Dee) saying (= homophone indicator) (‘dee’ = ‘d’ = D-) Charlie (-C-) is consumed by air miles (air miles), flying all over the place (= anagram indicator) (air miles = -IS-LAIMER) (DISCLAIMER),
7 Go ensure (go ensure) boss (= anagram indicator) (go ensure = GENEROUS) is not afraid to loosen the purse strings (GENEROUS),
13 Prepare for the competition (TRAIN) with one of going overseas (FERRY) (TRAIN FERRY) in hybrid form of transport (TRAIN FERRY),
14 Proposal (OFFER) for volunteer? (OFFER),
16 Fixed up Old Brogan (REHEELED fixed up old brogan/shoe) with grass (RE-ED) found around part of Italy (-HEEL- from part of Italy/ Heel of Italy) (REHEELED),
17 Ponders about (REFLECTS) fellow (-F-) student (learner = -L-) in secret (secret) movement (= anagram indicator) (secret = RE-ECTS) (REFLECTS),
19 One of those with pride (lion = L-ION) consuming good books (Old Testament = -OT-) (LOTION) that will presumably save one’s skin (LOTION),
21 Crumbles (ERODES) with seed or (seed or) nuts (= anagram indicator) (seed or = ERODES),
23 The Fringe’s backstage pass (the fringes ‘BAckstage paSS’ = BASS) is for member of barbershop quartet (BASS),
24 One of The Commotions (FLAP) turns up (= reversal indicator) for Bud (pal = -LAP) and loud music (forte = F-) (FLAP).