Across:
1 Raises (= reverse indicator) reduced (e) search party out west (posse) ('posse' without 'e' = poss = -SSOP) around Yarmouth (around 'YarmoutH' = yh = HY-) (HYSSOP) for an aromatic plant from mint family (HYSSOP),
4 Providing moral direction (EDIFYING) and energy (E-) to dignify (dignify = -DIFYING) deviation (= anagram indicator) (EDIFYING),
9 Tub removed from tributary (‘tub’ removed from ‘tributary’ = RARITY) – it’s a collector’s item (RARITY),
10 Dreads (FEARS-) losing second (‘lOsing’ second letter = -O-) game? That’s fairly (half) (half of ‘gaME’ = -ME) (FEARSOME) hair-raising! (FEARSOME),
12 Serve coffee, for instance (-POUR-), after (= positional indicator) drugs (POT-) fix (rig) – briefly (g) (‘rig’ without ‘g’ = -RI) (POTPOURRI), it changes the smell in the house (POTPOURRI),
13 Alley cats call out (‘alley cats’ call out/without ‘call’ = aeyts = YEAST) one of those responsible for The Rising (YEAST),
14 Most of those in 25 across (most of ‘pieces’ = PIECE) and nearly all those in 19 down (nearly all ‘eighth’ = EIGHT) following (= position indicator) order (-O-) to open fire (open/first letter of ‘Fire’ = -F) (PIECE OF EIGHT) in old Spanish capital (old Spanish capital/money = ‘Spanish dollar’ = PIECE OF EIGHT),
18 Vet (vet/check = -EXAMINE) in a bad mood (CROSS-) is the first (= position indicator) (CROSS -EXAMINE) to question the evidence in court (CROSS-EXAMINE),
21 Paramilitaries (IRA-) reject (= reversal indicator) measure of intelligence (IQ = -QI) (IRAQI) from Middle East resident (IRAQI),
22 Ongoing (INCESSANT) instances (instances) all over the place (= anagram indicator) (instances = INCESSANT),
24 Instruments (HELICONS) in The Inferno (hell) – for the most part (l) (‘hell’ without ‘l’ = HEL-) from Second Circle (second ‘cIrcle’ = -I-) with criminals (-CONS) (HELICONS),
25 Bits (PIECES) he’d removed from cheese dip (‘hed’ removed from ‘cheese dip’ = ceseip = PIECES),
26 The big pitch (HARD SELL) is tough as old boots (HARD) to deal with (SELL) (HARD SELL),
27 Ducks (ELUDES) howling coming from dwelling house (‘howling’ coming from ‘dwelling house’ = deluse = ELUDES).
Down:
1 Noisy type in car (HORN-) takes channel (-PIPE) (HORNPIPE) to overseas dance (HORNPIPE dance for sailors),
2 Small (S-) award (-PRI-ZE-) over time (-T-) for registered (-R) (SPRITZER) fizzy drink (SPRITZER),
3 Better (OUTDO) to party (-DO) on holiday (OUT-) from the start (= positional indicator) (OUTDO),
5 Historically speaking, Stagecoach (stagecoach vehicle = DILIGENCE) ultimately (= position indicator) is outstanding (DUE) (DUE DILIGENCE) – a concern for anyone doing preparatory work on Wall Street? (DUE DILIGENCE),
6 Soft hairy (soft hairy) bats (= anagram indicator) (soft hairy = FORSYTHIA) found in the gardens of China (FORSYTHIA),
7 Tips in service of (tips/first letters ‘In Service Of’ = ISO-) public house (-BAR) (ISOBAR) – The Front Line? (line indicating a weather front = ISOBAR),
8 Got the (got the) doctor (= anagram indicator) (got the = GHETTO) for those in rundown part of the city (GHETTO),
11 Career (PROFESSION-) for gangster (Al Capone = -AL) (PROFESSIONAL) and one getting paid for shooting, for instance (PROFESSIONAL),
15 Those listening out for sounds (EARPIECES) by the canals, for example? (ear canal / EARPIECES),
16 Loud (forte = F-), wild (= anagram indicator) canine (canine = -INANCE-) Old Penny (-D) (FINANCED) paid for (FINANCED),
17 Those working on bridges (DENTISTS) produced foolish (= anagram indicator) dissent (dissent = DEN-ISTS) over central keystones (central ‘keysTones’ = -T-) (DENTISTS),
19 Percentage (EIGHTH) of lighthouses with no souls? (‘lighthouses’ with no ‘souls’ = ighthe = EIGHTH),
20 Editorials dismiss diet (‘editorials’ dismiss ‘diet’ = orials = SAILOR) with salt? (old salt = SAILOR),
23 Close to Naas (close/end to ‘naaS’ = S-) – a town in Kildare (-KILL) (SKILL) with expertise (SKILL).