Across:
1 No menace in trace element (no ‘menace’ in ‘trace element’ = trelet = LETTER) that came in the post (LETTER),
4 Unlawful (CRIMINAL) power taken away from imperial crown (‘power’ taken away from ‘imperial crown’ = imialcrn = CRIMINAL),
9 In essence (NATURE), the banks’ national treasure (the banks ‘national treasure’ = NATURE),
10 Criminal (= anagram indicator) CID react (CID react = ACCREDIT) to give authority to the situation (ACCREDIT),
12 With all the seats taken in The Dáil (FULL HOUSE), it gives one the upper hand perhaps (FULL HOUSE in poker),
13 Supporter of the painter (EASEL) in artist’s studio (EASEL),
14 A job for a snoop (LETTER OPENER) in 1 across (‘letter’ = LETTER) and 19 down (‘opener’ = OPENER) (LETTER OPENER),
18 Correspondent (LETTER WRITER) opens 14 across (‘letter opener’ = LETTER) and 27 across (‘writer’ = WRITER) (LETTER WRITER),
21 Instrument (PIANO) for pain (pain = PIAN-) management (= anagram indicator) osteopath introduced (‘osteopath’ introduced = -O) (PIANO),
22 Honouring (ENNOBLING) Nigel (Nigel = E-LING) travelling (= anagram indicator) around German city (Bonn = -NNOB-) the wrong way around (= reverse indicator) (ENNOBLING),
24 Foolish type (NUMSKULL) is almost (b) frozen (‘numb’ without ‘b’ = NUM-) protecting The Head (-SKULL) (NUMSKULL),
25 Turns up (= reversal indicator), young man (lad = D-AL) holding onto energy (-E-) finds brief (w) quarrel (‘row’ without ‘w’ = ro = OR-) (ORDEAL) an unpleasant experience (ORDEAL),
26 Writing (R-) to paramilitaries (IRA = -ARI-) in retrospect (= reversal indicator) with affiliations (-TIES) (RARITIES) to very uncommon types (RARITIES),
27 Warrant (WRIT-) for that woman (her) losing the head (h) (‘her’ without ‘h’ = -ER) (WRITER) reported in the paper (WRITER).
Down:
1 Approaching the end of the journey, what the crew are expecting to see (LANDFALL) from earthmover perhaps? (LANDFALL),
2 It means everything (TOTALITY) to Count (TOT-) travelling (= anagram indicator) Italy (Italy = -ALITY) (TOTALITY),
3 Where to find everyone (EARTH) with heart (heart = EARTH) problem (= anagram indicator),
5 Tennis equipment (RACKET-) from England (-E-) and Spain (-E-) used by boxers (-RING) (RACKETEERING) working with criminals (RACKETEERING),
6 Pusher (MARKETEER) in commercial area (MARKET-) with drugs (E+E = -EE-) to get head right (head ‘right’ = -R) (MARKETEER),
7 Has nothing on (NUDISM) the philosophy that discards everything (NUDISM),
8 Lose one’s temper with someone (LET FLY) with sling (LET FLY),
11 Unexpectedly (OUT OF THE BLUE), it’s no longer popular (OUT) with old (O-), loud (forte = -F), sad music (the blues) for the most part (s) (‘the blues’ without ‘s’ = THE BLUE) (OUT OF THE BLUE),
15 Hold’s the viewer’s attention presumably (EYE SOCKET) like a hole in the head (EYE SOCKET),
16 Emailed (S-ENT) over tax (DIRT tax) return = (reverse indicator) (DIRT = -TRID-) (STRIDENT) – it’s offensively loud! (STRIDENT),
17 One can’t stay still (WRIGGLER) with river (-R) snake (WRIGGLE-) from the start (= position indicator) (WRIGGLER),
19 First act (OPENER) of Peron (Peron = OP-NER) revolution (= anagram indicator) included Evita from the start (‘Evita’ from the start = -E-) (OPENER),
20 Not as cold (WARMER) in snug (WARM-) with Cockney girl (’er = -ER) (WARMER),
23 Serbo-Croat dismisses a Scot (‘Serbo-Croat’ dismisses ‘a Scot’ = erbor = BORER) with drill (BORER).