Across:
1 Is alone (SINGLE) coming out of the chipper (SINGLE),
4 One of those containing 1 down (SCOTLAND containing ‘Scottish’) in the country (SCOTLAND),
9 A tenor (a tenor) arrangement (= anagram indicator) (a tenor = ORNATE) is over-elaborate (ORNATE),
10 A gun (a gun) goes off (= anagram indicator) (a gun = -UNGA-) in Knock (blow = B-LOW) (BUNGALOW) cottage (BUNGALOW),
12 What’s highly classified? (TOP SECRET) God knows! (TOP SECRET),
13 Valet leaves travelogue (‘valet’ leaves ‘travelogue’ = rogue = ROGUE) with scoundrel (ROGUE),
14 18 across opens (‘single whisky’ opens = SINGLE) page (P-) from Norse (Norse = -ERSON) novel (= anagram indicator) (SINGLE PERSON) – one that’s yet to be matched (SINGLE PERSON),
18 One of the spirits (SINGLE WHISKY) in 1 across (‘single’ = SINGLE) shocked (= anagram indicator) Sikh (sikh = -HISK-) in Wyoming (WY= W-Y) (SINGLE WHISKY),
21 Set off from near east (‘set’ off from ‘near east’ = neara = ARENA) for Croke Park, for one (ARENA),
22 Infamous sort (NOTORIOUS) of leaders from number of (leaders from ‘Number Of’ = NO-) states (= homophone indicator) – two (‘two’ = ‘to’ = TO-) in Brazil (-RIO-) and America (-US) (NOTORIOUS),
24 In favour of (PRO-) saying hello (-HI-) to section (-BIT) (PROHIBIT) of bar (PROHIBIT),
25 Sailor (Able-Bodied seaman = AB-) is the type of person taking advantage (-USER) (ABUSER) of addict (ABUSER),
26 To see the world (travel) mostly (l) (‘travel’ without ‘l’ = TRAVE-) from a messy, beastly place (-STY) (TRAVESTY) is a poor substitute for the real thing (TRAVESTY),
27 Unprecedented events (FIRSTS) concealed by anarchist’s riflemen in revolution (= reverse indicator) (concealed by ‘anarchiSTS RIFlemen’ = stsrif = FIRSTS).
Down:
1 Small (S-) crib (-COT-) with this (this) problem (= anagram indicator) (this = -TISH) (SCOTTISH) for one lot of the Celts (SCOTTISH),
2 What you’d be left with in Connemara National Park if three went missing (Connemara National Park Twelve Pins with three missing = nine pins = NINEPINS) and were found down in the bowling alley? (NINEPINS / type of bowling),
3 Two articles (the + the), in two different languages (‘the’ French’ = LA- + ‘the’ English = -THE= LATHE) picked up by one working on deal? (carpenter working on deal wood with LATHE),
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 Followed Celtic in Ireland in the boom years (Celtic TIGER) with Dublin girlfriend (MOTH) (TIGER MOTH) – one of the flighty de Havillands? (Geoffrey de Havilland designed the TIGER MOTH aeroplane),
7 Bear witness to (ALLEGE) sons from Los Angeles (‘sons’ from ‘Los Angeles’ = lagele= ALLEGE),
8 Presumably, he expects to find you well (locate ground water/well = DOWSER) and he’ll bring you to the watering hole (DOWSER),
11 Clip of (a) publicity material (propaganda) (‘propaganda’ without ‘a’ = PROPAGAND-) on one of those found in 27 across, in short (one ‘firsts’ = ‘first’ + in short = 1st = -IST) (PROPAGANDIST) by a spin doctor (PROPAGANDIST),
15 Saved (LAID ASIDE) the foreign (‘the’ French = LA-) papers (-ID papers) providing comment to the audience (ASIDE) (LAID ASIDE),
16 In the past, marries (ESPOUSES) sweetheart (‘swEet’ heart = E-) from the harem presumably (-SPOUSES) (ESPOUSES),
17 Study (EYE-) problems for the ramblers (-SORES) (EYESORES) in areas with too many ugly weeds (EYESORES),
19 Jaguar (CAR-) cat (-PET) (CARPET) in ground cover (CARPET),
20 Backing (= reverse indicator) Adam (-A) and Rod (rod = -DOR-) with electronic (-E-) loud music (forte = F-) (FEDORA) – the type that goes over the heads of men of a certain vintage! (FEDORA),
23 Teacher of religion (RABBI) recalls (= reverse indicator) Pole (bar = RAB-) with a fair amount of (half of) bias (half of ‘BIas’ = BI-) (RABBI).