Across:
1 Cover old ground (REHASH) with imp leaving Hampshire (‘imp’ leaving ‘Hampshire’ = hashre = REHASH),
4 Dish the dirt (SPEAK OUT) starting security (starting ‘Security’ = S-) summit? (-PEAK) That’s unacceptable! (OUT) (SPEAK OUT),
9 The Stones’ (GRAVEL) great (G-) composer (Maurice -RAVEL) (GRAVEL),
10 Miss (LEAVE OUT) Holiday (LEAVE) is no longer in fashion (OUT) (LEAVE OUT),
12 Bully (COW-) drunk (= anagram indicator) a cider (a cider = -ARDICE) (COWARDICE) and left before the fight (COWARDICE),
13 Hoodoo (spell / CURSE) similar to 3 down (‘swear’ is similar to CURSE),
14 New issues (DEVELOPMENTS) to improve (DEVELOP-) worker’s (workers = -MEN-) shirt (-T- shirt) with top-stitching (top ‘Stitching’ = -S) (DEVELOPMENTS),
18 Broadcast (TRANSMISSION) in part relates to petrol heads (car TRANSMISSION),
21 Po-faced (prim = PRI-M) about school (-S-) (PRISM) sending colourful types in a number of directions (PRISM),
22 Anyone can (anyone can) disguise (= anagram indicator) (anyone can = ANNOYANCE) irritation (ANNOYANCE),
24 Study (READ-) community foundation (‘communitY’ foundation = -Y) briefly (h) with (‘with’ without ‘h’ = ‘WIT’) (READY WIT) a sense of humour that’s off the cuff (READY WIT),
25 Sharp knock at the door (RAT-TAT) of opium dens outside studio apartment (‘opium dens’ outside ‘studio apartment’ = taartt = RAT-TAT),
26 It’s indicative of Holly showing age (TREE RING) starting trouble (starting ‘Trouble’ = T-) by lough (Lough Ree = -REE) in Waterford (RING) (TREE RING),
27 Show up (ATTEND) to lure out of laundrette (‘lure’ out of ‘laundrette’ = andtte = ATTEND).
Down:
1 What that murderous lot in The Palace did (REGICIDE) about (RE-) soldier (-GI-) not finishing (r) drink (‘cider’ without ‘r’ = -CIDE) (REGICIDE),
2 What The Sun’s fans are looking for (HEATWAVE) leads to local pressure from police department (local ‘pressure’ = HEAT) to renounce (waive) report? (= homophone indicator) (‘waive’ = ‘WAVE’) (HEATWAVE),
3 Promise to keep omertà (SWEAR) ultimately to Mafia’s (ultimately to ‘mafiaS’ = S-) Don (-WEAR) (SWEAR),
5 Agree in advance (PREDETERMINE) to briefly (p) get ready (‘prep’ without ‘p’ = PRE-) to put a stop to (-DETER-) quarry (-MINE) (PREDETERMINE),
6 Promoters (ADVOCATES) of Scottish silks (Scottish silks/Scottish barristers = ADVOCATES),
7 Smells (ODOURS) funny? (= anagram indicator) Ours do! (ours do = ODOURS),
8 Test pilot blows top (‘test pilot’ blows ‘top’ = estilt = TITLES) with a number of counts (a number of Counts = noble TITLES),
11 Detective’s (DIs = DIS-) record (Extended-Play record = EP) turning (= reversal indicator) (EP = -PE-) extreme agents (extreme ‘agentS’ = -S-) in Republic, for one (-N-ATION) (DISPENSATION), leads to immunity (DISPENSATION),
15 Turns up (= reverse indicator), Doctor (MD = -D M-) Rowe (rowe) fairly (half of) (half of ‘ROwe’ = ro = -OR-) welcomed by sovereign (royal = L-AYOR) (LORD MAYOR) – the first citizen? (LORD MAYOR),
16 Brief description (VIGNETTE) of engineer (= anagram indicator) vetting (vetting = VIGN-TTE) Oriental (East = -E-) inside (VIGNETTE),
17 Heads of university not (heads of ‘University Not’ = UN-) scrutinised (-TESTED) (UNTESTED) or is that not proven yet? (UNTESTED),
19 Staggers (trips) back (= reversal indicator) (trips = SPIR-T) around island (-I-) (SPIRIT) from the bar (SPIRIT),
20 Detective (PI-) and judge (-RATE) (PIRATE) working overseas with Grace O’Malley (PIRATE),
23 All those in favour (YEAS-) of the leading (‘The’ leading = -T) (YEAST) agent behind The Rising (YEAST).