Across:
8 Collaborates without locals (‘collaborates’ without ‘locals’ = borates = BOATER) out on the lake for pleasure (BOATER),
9 A number of instruments (WHISTLES) for closing 15 across (closing ‘blows the whistle’ = WHISTLE-) with sulphur (-S) (WHISTLES),
10 It comes as a disappointment (BLOW) to Hook (hook/dig = BLOW)
11 Bad feeling (ANTAGONISM) as gas main not (gas main not = ANTAGONISM) working (= anagram indicator),
12 Great (G-) scope (-RANGE) (GRANGE) for country house with farm buildings (GRANGE),
14 Before noon (-AM-) in a Mediterranean island (Crete = CRE- TE-), go for a (-A-) (CREAM TEA) speciality repast (CREAM TEA),
15 Exposes corruption (BLOWS THE WHISTLE) to 10 across (‘blow’ = BLOW-) on bender (-S bend) hearing (= homophone indicator) Hugh (‘Hugh’ = ‘hew’ = -HE W-) is consumed by weed (T- HISTLE)? (BLOWS THE WHISTLE),
18 Couple of beastly figures (ass + ass = ASSASS-) up (= reversal indicator) north (NI = -IN) (ASSASSIN) capable of getting away with murder (ASSASSIN),
20 Relative (AUNTIE) gets a (A-) release (-UNTIE) (AUNTIE),
22 Doing what those in 24 across are capable of (‘refs’ are capable of REFEREEING) is like doing work in Ring (doing work in boxing ring = REFEREEING),
24 Does a similar job in 15 across (“blows the whistle” = REFS) for the character witnesses (referees), in short (REFS),
25 Pressure (P-) to receive (-ERCEIVE) order (= anagram indicator) (PERCEIVE) from judge (PERCEIVE),
26 In part, Christian Gershwin (in part ‘Christian Gershwin’ = ANGERS) rubs people up the wrong way (ANGERS).
Down:
1 Upsets (= reversal indicator) everyone (all = -LLA-) in bar (rod = DO-R) (DOLLAR) in foreign capital (foreign capital/money = DOLLAR),
2 Ned is not in the West End (“Ned” is not in the “West End” = west = STEW) for the dinner (STEW),
3 Ran to men (ran to men = ORNAMENT) working (= anagram indicator) in The Cabinet (ORNAMENT),
4 Powerful blow (SWAT) found in Washington’s Watergate (found in ‘Washington’s Watergate’ SWAT),
5 It tends to pull some strings for product placement (JINGLE) in The Sound of Music (JINGLE),
6 Tablet (STONE-) is for two family members (ma + son= MASON) (STONEMASON) and one of the chisellers (STONEMASON),
7 Footage (NEWSREEL) of modern (NEW-) school (-S-) sounds (= homophone indicator) genuine (“real” = “reel” = -REEL) (NEWSREEL),
13 Half of what’s in 7 down (half of ‘newsreel’ = NEWS) working on the campaign (working on the ‘ad’ campaign / AGENCY) (NEWS AGENCY) for Reuters (NEWS AGENCY),
14 Sweetheart (‘sweet’ heart = -E-) in family (CL-AN) (CLEAN) is free of drugs (CLEAN),
16 One of those with bug (LISTENER) let siren (let siren = LISTENER) off (= anagram indicator),
17 Teacher (HEAD-) with a number of gowns perhaps (-GEAR) (HEADGEAR) and mortarboards (HEADGEAR),
19 Pretty as a picture (SCENIC) since (since = SCENI-) changing (= anagram indicator) college (-C) (SCENIC),
21 Initial flavour (initial ‘flavour’ = -F-) in exotic (= anagram indicator) Mirin (Mirin = IN-IRM) (INFIRM) is weak (INFIRM),
23 I dare say, Ray’s missing (‘I dare say’ ‘Rays’ missing / without ‘rays’ = idea = IDEA) in theory (IDEA),
24 King Lear has no link (‘King Lear’ has no ‘link’ = gear = RAGE) to show of displeasure (RAGE).