Across:
1 Confront (TACKLE) smart aleck dropping arms ('smart aleck' letting off 'arms' = atleck = TACKLE).
4 Spies (spies) all over the place (= anagram indicator) (spies = EPIS-S) hiding code (code) beginning to go off (c) (‘code’ without ‘c’ = -ODE) (EPISODES) in instalments (EPISODES),
9 Removing principal (principal = first in order = t) from public address system (‘tannoy’ without ‘t’ = ANNOY-) at school (-S) (ANNOYS) gets on someone’s nerves (ANNOYS),
10 Detain my (detain my) butcher (= anagram indicator) (detain my = DYNAMITE) – the one with the short fuse perhaps (DYNAMITE),
12 Suspect (SMELL A RAT) Old Bob (old bob = shilling = S-) Melvin (-MEL-) lost (-L) place in Meath (Tara) going all the way back (= reverse indicator) (Tara = A RAT) (SMELL A RAT),
13 Unseen on the outskirts of (‘UnseeN’ on the outskirts of = UN-) Hanover (Hanover knot/ tie = -TIE) for free (UNTIE),
14 What’s left of late (ESTATE) by a (A-) lady’s man (-GENT-) to auspicious foundation (‘auspiciouS’ foundation = -S) (ESTATE AGENTS) accommodating the homeless? (ESTATE AGENTS),
18 According to the Americans, looks after Czech (Czech) State (= homophone indicator) (Americans looks after ‘Czech’ = Americans looks after ‘check’ = FOOTS THE BILL) and pays for it (FOOTS THE BILL),
21 Switches (= reversal indicator) a (-A) precious stone (gem = -MEG-) for ring (O-) (OMEGA) – the last in a series (the last letter of Greek alphabet = OMEGA),
22 Warrior (ECO- warrior) is not (not = -NO-T) over young woman (miss), briefly (s) (‘miss’ without ‘s’ = -MIS-) (ECONOMIST) employed as an expert on financial management (ECONOMIST),
24 Local pump attendant (barman = BAR-MAN) carries, for example (e.g. = -GE-), a round (‘a roung’ = ‘around’ = reverse indicator) (BARGEMAN) for one of those who owns a lighter? (BARGEMAN owns a lighter / a barge),
25 Talking points perhaps (ROSTRA) in cirrostratus (in ‘cirROSTRAtus’ = ROSTRA),
26 Pass (OVERTAKE) five (-V-) submerged in chopped up (= anagram indicator) oak tree (oak tree = O-ERTAKE) (OVERTAKE),
27 Picks up (HEARS-) energy (-E) (HEARSE) from the type going on a final journey of late (HEARSE).
Down:
1 Fled (-RAN-) after (position indicator) time (T-) with whistle-blowers (refs) sending up (= reverse indicator) (refs = -SFER) (TRANSFER) Movement (TRANSFER),
2 Scot sent (scot sent) off (= anagram indicator) (scot sent = CONTESTS) in a number of matches (CONTESTS),
3 Characters in Molloy, a literary (characters in ‘molLOY, A Literary’ = LOYAL) offering that’s faithful to others (LOYAL),
5 Be really careful (PAY ATTENTION) or get punished (PAY) for taking treatment (ATTENTION) (PAY ATTENTION),
6 Alien (-ET-) caught in the act (the act/written law = statute = STATU-TE) (STATUETTE) by Oscar, for one (Oscar is a STATUETTE given as an Academy Award),
7 Small, pretty (DAINTY) local dinky (local ‘dinky’ = DAINTY),
8 Wild (= anagram indicator) weeds (weeds = SWEDE-) at the end of garden (end of ‘gardeN’ = -N-) (SWEDEN) in the country (SWEDEN),
11 Pay no heed to budget (BREAK THE BANK) for short holiday (BREAK) beside The Lee (THE BANK of the River Lee) (BREAK THE BANK),
15 When one didn’t stand (TOOK A SEAT) how did one get elected? (TOOK A SEAT),
16 Evil looking (SINISTER) hospital worker (sister = S-ISTER) doing the rounds at home (at home = -IN-) (SINISTER),
17 Likes to share (FLATMATE) stout (F-AT-) including port (left = -L-) and Bud? (-MATE) (FLATMATE)
19 A mistake (BOO-BOO) not finishing (k + k) two novels, one after another (Book + Book) (‘book’ and ‘book’ without ‘k and ‘k’ = BOO-BOO),
20 Trade secret – releasing tarts (‘trade secret’ – releasing ‘tarts’ = deecre = DECREE) to order (DECREE),
23 It’s the wizard’s place (OZ-) to lose the head (t) on pitch (‘tone’ without ‘t’ = -ONE) (OZONE) – and that can be toxic in the upper echelons (OZONE).