Across:
8 Writer (PENCIL) starts on 15 across (starts on ‘pencil sharpener’ = PENCIL),
9 Boss (= anagram indicator) trained (trained = RATI-NED) around Oxford (-O-) (RATIONED) passed out the allowance (RATIONED),
10 Serves up (= reversal indicator) sample of good nibbles (sample of ‘good nibbles’ = d nib = BIND) with bit of a pickle (BIND),
11 Arguing (ROWING) with snaky type (BOA-) starting trouble (starting ‘trouble’ = -T) (ROWING BOAT) and going overseas (ROWING BOAT),
12 Sultan is missing from the publication’s (‘Sultan’ is missing from the ‘publications’ = pbicio = BIOPIC) autobiographical film (BIOPIC),
14 Fellow (ACADEMIC) produced a (A-) computer-aided design (computer-aided design = -CAD-) to train (= anagram indicator) mice (mice = -EMIC) (ACADEMIC),
15 Takes head off writer (PENCIL SHARPENER) for making a point (PENCIL SHARPENER),
18 Targets a (targets a = REGATTAS) swimming (= anagram indicator) series of contests on the water (REGATTAS),
20 Possessions (ASSETS) from a (A-) place (-SET-) in the boat (-S-S) (ASSETS),
22 Play (= anagram indicator) the (the = ETH-) single (-I-) with introduction on (introduction ‘on’ = -O-) short (o) instrument (‘piano’ without ‘o’ = PIAN-) taking direction (south = -S) (ETHIOPIANS) from Sudanese neighbours (ETHIOPIANS),
24 Master (GURU) golf (G-) at old city (-UR-) university (-U) (GURU),
25 Come together (COALESCE) for company (CO-) with hotshot (-A-CE) welcoming the French (the = -LES-) (COALESCE),
26 Take one back (REMIND one) for short (i) dress (‘mini’ without ‘i’ = -MIN-) in red (RE-D) (REMIND),
Down:
1 Gets to operate perhaps (MEDIC-) on one (-I) (MEDICI) of the old Italian moneylenders (MEDICI family of bankers),
2 Drug (ACID) to medicate characters (‘medicate’ characters = dica) cut back? (= reversal indicator) (‘dica’ = ACID),
3 Minister (CLERIC-) gets a (-A-) tip for lots (tip for ‘lots’ = -L) (CLERICAL) of office work (CLERICAL),
4 Raise (= reversal indicator) sample of cornflower blue (sample of ‘cornflower blue’ = wer b = BREW) to make a drink (BREW),
5 Curtailed part of (d) the stadium (‘stand’ without ‘d’ = STAN-) with mid-sized (mid/centre of ‘sized’ = -Z-) area (-A) (STANZA) for the linesman to workout (STANZA),
6 Two measures at the bar (DOUBLE) for a selection of Rosés, for instance (BEDS for roses) (DOUBLE BEDS), for a number of the sleeping partners (DOUBLE BEDS),
7 Not showing enthusiasm (NEGATIVE) for new (= anagram indicator) vintage (vintage = NEGATIV-) from Spain (-E) (NEGATIVE),
13 Slight fault (PECCADILLO) serving up (= reversal indicator) mushroom (cep = PEC-) with head of cauliflower (head of ‘cauliflower’ = -C-) and a (-A-) herb (-DILL-) with duck (-O) (PECCADILLO),
14 Perhaps a grateful (ASHES from the grate/a grateful) tout from south-east (‘tout’ from ‘south-east’ = soeas = ASHES),
16 Apply pressure (EXERT-) to one of those charged (-ION) (EXERTION) in Endeavour (EXERTION),
17 Comfort (REASSURE) from Erasure’s (erasures = REASSURE) melody (= anagram indicator),
19 One doing a job for “The Pool” (TYPIST from typing pool) isn’t likely to put pen to paper in the office (TYPIST),
21 Presumably, seating area in The Palace (THRONE) where member of royal family relaxes watching The Queen (THRONE),
23 Vines from expansive (‘vines’ from ‘expansive’ = expa = APEX) summit on the mountain, for example (APEX),
24 Some forthcoming amendment (some “forthcoming amendment” = g ame = GAME) could be a distraction (GAME).