Across:
1 Samples (PIECES) those at the end of 18 across (at end of 'museum pieces' = PIECES),
4 It’s perfect for meringue (EGG WHITE), for example (e.g. = EG-), and for shifting (= anagram indicator) weight (weight = -G WHITE) (EGG WHITE),
9 Tips in service of (tips/first letters ‘In Service Of’ = ISO-) public house (-BAR) (ISOBAR) – The Front Line? (line indicating a weather front = ISOBAR),
10 Lots of praise (OVATIONS) as zero (O-) tax (-VAT-) is charged (-ION-) for activities at foundation (‘activitieS’ at foundation = -S) (OVATIONS),
12 Pasta (CARBONARA) produced by jazz (= anagram indicator) bar (bar = -ARB-) with duck (-O-) took off (ran = -NAR-) over (= reverse indicator) in California (CA = C-A) (CARBONARA),
13 Lift (RAISE) is (-IS-) in the middle of elevated (= reversal indicator) space for The Hearing (ear = RA-E) (RAISE),
14 One of those in police station (SUPER-) has not been processed (-NATURAL) (SUPERNATURAL) – it’s a concern for those who think the spooks are involved (SUPERNATURAL),
18 The bare bones of our natural history perhaps (MUSEUM PIECES) to ponder (MUSE-) at university (-U-) on Medium-Sized (-M-) Allotments (PIECES) (MUSEUM PIECES),
21 Had a row (OARED) even with local (even letters with ‘lOcAl’ = OA-) revolutionary (-RED) (OARED),
22 Exceptional people (RARE BIRDS) in the glossy Ibises (‘glossy ibises’ are RARE BIRDS / and could be glossy Ibis hotels),
24 When you lose energy (POWER CUT), expect blackouts with this (POWER CUT),
25 Opening unctuous (opening ‘Unctuous’ = U-) wines (wines = -NWISE) all over the place (= anagram indicator) (UNWISE) is reckless (UNWISE),
26 Elizabeth (LILY) and Bill (-AD-) behind (= position indicator) post (P-) and fences at The Foundation (‘fenceS’ at the foundation = -S) (LILY PADS) leading to leaves on the pond (LILY PADS),
27 A hat (BONNET) for non-inheritable declining hairline? (‘non-inheritable’ declining/without ‘hairline’ = nontbe = BONNET).
Down:
1 Criminal (= anagram indicator) snipers (snipers = PRIN-ESS) caught (-C-) inside (PRINCESS) might still have a shot at the monarchy perhaps? (PRINCESS),
2 Presumably, self-catering holidays (EGO TRIPS) for those movements that appeal to narcissists (EGO TRIPS),
3 A source of inspiration (muse of lyrical poetry / source of inspiration = ERATO) going around (= reverse indicator) some chariot arena (some ‘chariOT AREna’ = otare = ERATO),
5 Declare briefly (GIVE A SUMMARY), I’ve (-IVE) been taken in by retiring (= reverse indicator) head barman (Attorney General = AG = G- A) with, it’s said (= homophone indicator), some (‘some’ = ‘sum’ = SUM-) woman’s name (-MARY) (GIVE A SUMMARY),
6 Providing a service, of course, for travellers in Venice (WATER TAXI), foreign (= anagram indicator) waiter (waiter = WATER -I) welcomes Levi (Levi) loudly (= homophone indicator) (‘Levi’ = ‘levy’ = TAX-) (WATER TAXI),
7 It’s symbolic (ICONIC) of one (I-) Cricket Club (CC = -C-C) touring around (-O-) Ulster (-NI-) (ICONIC),
8 Followed (ENSUED) workers (men) tip-off (m) (‘men’ without ‘m’ = EN-) – it’s what the litigant did (-SUED) (ENSUED)
11 Motorcade (PARADE) for Clay? (GROUND) (PARADE GROUND) Those trained to fight are expected to leave their stamp on it presumably! (PARADE GROUND),
15 This will bring The Tourists back to where it all started (ROUND TRIP) and it’s in The Ticket perhaps? (ROUND TRIP ticket),
16 It has a sting in its tail (SCORPION) for detective (-PI-) found in contempt (scorn = SCOR-N) over taking the lead (‘Over’ taking the lead/first letter = -O-) (SCORPION),
17 Easiest (easiest) way (= anagram indicator) (easiest = AS I SEE -T) around island (-I-) (AS I SEE IT), if you ask me? (if you ask me = AS I SEE IT),
19 Put the screws on (COMPEL) firm (CO-), marble-top (‘Marble’ top = -M-) gym (-PE-) locker for beginner (‘Locker’ for beginner/first letter = -L) (COMPEL),
20 Brickie’s tool (TROWEL) and mask from metalworks (‘mask’ from ‘metalworks’ = etlwor = TROWEL),
23 Remove salt from longboats (remove ‘salt’ from ‘longboats’ = ongbo = BONGO) in drums (BONGO drums).