Across:
8 Double agents don’t get stoned (‘double agents’ don’t get/without ‘stoned’ = ublage = BELUGA) in the expensive fish restaurant (BELUGA caviar),
9 English neighbour (SCOTSMAN) with mascots (mascots = SCOTSMA-) out (= anagram indicator) with Dublin side (‘Dublin’ side = -N) (SCOTSMAN),
10 Difficult (HARD) for daughters to get rid of guest (‘daughters’ get rid of ‘guest’ = dahr = HARD),
11 Dismissed (DISCHARGED) detectives (DI’s = DIS-) accused in the station (-CHARGED) (DISCHARGED),
12 Head off (w) to war (war’ without ‘w’ = AR-) on unfinished (l) waterway (‘canal’ without ‘l’ = -CANA) (ARCANA) with mysterious types (ARCANA),
14 Changed one’s mind (RECANTED) reading (R-) English- (-E-) -German philosopher (‘Kant’ = ‘cant’ -CANT-) to the audience (= homophone indicator) for editor (-ED) (RECANTED),
15 Ring (TELEPHONE) is one example (NUMBER) (TELEPHONE NUMBER) of the sort kept in a cell in America (TELEPHONE NUMBER kept in a cell/phone in America),
18 Leave it (BEQUEATH), as it’s fairly (half of) unique (half of ‘unique’ = -QUE-) in strike (beat = BE-AT-) march (march) to The Foundation (h) (‘march’ to the foundation = -H) (BEQUEATH),
20 Most of the (most of ‘the’ = TH-) regiment (-R-) is not well (-ILL) (THRILL) in Rush (rush/buzz = THRILL),
22 The sparkling stuff (GLITTER-) could be Asti (Asti) excluding some initially (‘some’ initially = s) (‘Asti’ without ‘s’ = -ATI) (GLITTERATI) for the major celebrities (GLITTERATI),
24 Tries on leaving chain stores (‘tries on’ leaving ‘chain stores’ = chas = CASH) in the capital (capital/money = CASH).
25 Picked out (ISOLATED) unfamiliar (= anagram indicator) details (details = IS-LATED) about Ring (-O-) (ISOLATED),
26 Odd (= anagram indicator) menu (menu = NU-ME-) includes tea (-T-) with little measure (gram = -G) (NUTMEG) of spice (NUTMEG).
Down:
1 In retrospect (= reversal indicator), more mature (riper = REP-IR) about a (-A-) (REPAIR) service (REPAIR),
2 One of those types with cauliflower head (CURD) puts up (= reversal indicator) note (music note -D) on old police force (Royal Ulster Constabulary = RUC = CUR-) (CURD),
3 Change (HARD CASH) what’s in 10 across (‘hard’ = HARD) and 24 across (‘cash’ = CASH) (HARD CASH),
4 Contemplates (muses) losing the head (m) (“muses” without “m” = USES) with exercises (USES),
5 Leave (GO-) most of (e) The (‘the’ without ‘e’= -TH-) Independent (-I-) copy heading (‘copy’ heading = -C) (GOTHIC) in old language (GOTHIC),
6 Night-watchman (ASTRONOMER) spends a lot of time looking into space (ASTRONOMER),
7 Seeing the vision without any outside interference (NAKED EYE), Yankee (Yankee = NAK- EYE) upset (= anagram indicator) over writer (-ED) (NAKED EYE),
13 Is it satisfactory (ADEQUATE-) place (lay) is empty (empty/centre removed: ‘a’) (‘lay’ without ‘a’= -LY) (ADEQUATELY) to an acceptable extent? (ADEQUATELY),
14 The Hacienda (RANCH) handled (RAN-) two types of drugs (cocaine and heroin = C + H = -CH) (RANCH),
16 Watch (EYE -) golf (-G-) with young woman (-LASS) (EYEGLASS) – that’s a solitary spectacle? (EYEGLASS),
17 Fool (NUT-) torn (rip/rent = -R-ENT) over one (-I-) (NUTRIENT) very good piece of food (NUTRIENT),
19 Spies (AGENTS) American (A-) reporting (= homophone indicator) Lou (‘Lou’ = ‘loo’ = -GENTS) (AGENTS),
21 Temporary occupant (LESSEE) of port (port/left = L-) city in Germany (Essen) hasn’t finished (n) (‘Essen’ without ‘n’ = -ESSE-) engineering (-E) (LESSEE),
23 Isolated without lost (‘isolated’ without ‘lost’ = iaed = AIDE) member of staff (AIDE),
24 Rome, for one (CITY), and Milan sacked by militancy (‘Milan’ sacked by/taken from ‘militancy’ = itcy = CITY).