Across:
1 View (ASPECT) of a (A-) small (-S-) leaderless (O) bunch of oil producers (‘OPEC’ without ‘O’= -PEC-) from half of Texas (half of ‘TX’ = -T) (ASPECT),
4 They’re synonymous with Blade Runner (ICE RINKS), as they can give those on the edge the slip (ICE RINKS),
9 Nicest (nicest = INSECT) hybrid (= anagram indicator) or Beetle (INSECT),
10 It’s used to damage the reputation of a person (MUD-) in the police force (-GUARD) (MUDGUARD) – the biker uses it as a defence (MUDGUARD),
12 What’s highly classified? (TOP SECRET) God knows! (TOP SECRET),
13 Good (good/moral = NOBLE) Lord! (NOBLE),
14 Page (P-) from Norse (Norse = -ERSON) novel (= anagram indicator) opening (= positional indicator) with introduction from 18 across (‘single whisky’ = SINGLE) (SINGLE PERSON) has yet to meet match (SINGLE PERSON),
18 One of the spirits (SINGLE WHISKY) in 20 down (‘single’ = SINGLE) troubled (= anagram indicator) Sikh (sikh = -HISK-) in Wyoming (WY= W-Y) (SINGLE WHISKY),
21 In Barcelona, so long (Spanish ‘so long’ = ADIOIS) as there is no malice in social media (there is no ‘malice’ in ‘social media’ = sodia = ADIOS),
22 Infamous type (NOTORIOUS) is onto our (is onto our = NOTORIOUS) mistake (= anagram indicator),
24 Got a job at a club (SIGNED) that’s popular (IN) (SIGNED IN) and on arrival did this for receptionist (SIGNED IN),
25 Sailor (able-bodied seaman = AB-) finds drugs (-USER) (ABUSER) for addict (ABUSER),
26 Sends back to the house (RE-ELECTS) for the Spanish (the = -EL-) involved in secret (secret = -RE-ECTS) arrangement (= anagram indicator) (RE-ELECTS),
27 Irish men (Irish ‘men’ = FIR-) taking car (model -T-) in ship (-S-S) (FIRSTS) for unprecedented events (FIRSTS).
Down:
1 Anonymous (A-) police department (Criminal Investigation Department = -CID) experiment (TEST) (ACID TEST) will conclusively tell you everything you need to know (ACID TEST),
2 Put off (POSTPONE) by all the letters (POST-) not (= anagram indicator) open (open = -PONE) (POSTPONE),
3 Seasonal work (CYCLE) for the pedlars (for the ‘pedlars’ = for the ‘pedallers’ = CYCLE) in a manner of speaking (= homophone indicator),
5 Stall (COUNTER-) at Writer’s Centre (‘writers’ centre = -T-) in Irish festival (Irish festival = feis = -FEI-S) (COUNTERFEITS) makes money (makes money= COUNTERFEITS),
6 Curry (ROGAN JOSH) took off (ran = R-AN) around track (dog track) that needs no introduction (d) (‘dog’ without ‘d’ = -OG-) with kid (kid/tease = JOSH) (ROGAN JOSH),
7 Barney (Barney = NEARBY) Duff (= anagram indicator) could be handy (NEARBY),
8 Endless (s) American beer (‘suds’ without ‘s’ = SUD-) for The Lions (-DEN) (SUDDEN) is unexpected (SUDDEN),
11 Major promoter (PROPAGANDIST) of Druid (-PAGAN-) is after (= positional indicator) professional (PRO-) director (-D-) who’s a winner (first = 1ST = -IST) (PROPAGANDIST),
15 Sounds like (= homophone indicator) staff members (‘aides’ = ‘aids’ = -AID S-) in Belgium city (Liege = L- -IEGE) (LAID SIEGE) attempted blockade of Rome, for instance (LAID SIEGE),
16 Favours (ESPOUSES) sweetheart (‘sweet’ heart = E-) from the harem presumably (-SPOUSES) (ESPOUSES),
17 Look out for (EYE-) wild (= anagram indicator) roses (roses = -SORES) (EYESORES) from areas with too many ugly weeds (EYESORES),
19 Scare a (scare a = CAESAR) crazy (= anagram indicator) emperor (CAESAR),
20 Is alone (SINGLE) coming out of the chipper (SINGLE),
23 Talk at length trivially (rabbit), for the most part (t) (‘rabbit’ without ‘t’ = RABBI), to religious teacher (RABBI).