Across:
1 Disciplines (disciplines/fields of expertise = FIELDS) those starting 18 across (starting ‘field marshal’ = FIELD-) on bender (-S bend) (FIELDS),
4 Wild (= anagram indicator) toucans (toucans = AC-OUNTS) enveloping front covers (front ‘Covers’ = -C-) (ACCOUNTS) of books (ACCOUNTS),
9 Belted out (YELLED) by women’s magazine (-ELLE-) following (= position indicator) story ultimately (‘storY’ ultimately = Y-) from the top-down (top ‘Down’ = -D) (YELLED),
10 Unsettled (= anagram indicator) a dreamer (a dreamer = DEMERARA)? Sugar! (DEMERARA sugar),
12 River rose (river rose) all over the place (= anagram indicator) (river rose = RESERVOIR) coming from artificial lake (RESERVOIR),
13 Book of maps (ATLAS) for Argentine province (Salta) going all the way back (= reverse indicator) (Salta = ATLAS),
14 Presumably, if you do drugs you’ll need this (PRESCRIPTION) to give to drug dealer for supply (PRESCRIPTION),
18 Rank (FIELD MARSHAL) piece of grassland (FIELD) and bog (MARSH-) close to a (-A-) port (port/left = -L) (FIELD MARSHAL),
21 Acknowledges (AVOWS) Woodcarvers released record (‘Woodcarvers’ released ‘record’ = woavs = AVOWS),
22 Popular (IN-) retweet (RT) sends up (= reverse indicator) (RT = -TR-) the German (‘the’ German = -DER-) in America (US = U-S) (INTRUDERS) and those who entered country illegally (INTRUDERS),
24 Educationalist (Sir = S-IR-) over conservative (-C-) Oxford (-O-) College (-C-) is fairly (half of) cool (half of ‘COol’ = -CO) (SCIROCCO) with hot type that blows in from Africa (SCIROCCO),
25 Plant (set = S-ET) found around alpine summit of (‘Alpine’ summit of = -A-) Switzerland (-CH-) (SACHET) produces a bag that contains a fragrant scent (SACHET),
26 Spies (moles = MOL-ES) surrounded a (-A-) ship (-SS-) (MOLASSES) with ingredients for rum (MOLASSES),
27 On the phone (KEYPAD) to sweetheart (‘swEet’ heart = -E-) in Kerry (K-Y-) with writer’s block? (-PAD) (KEYPAD).
Down:
1 Charge (FLY-) unit (part = -TRAP-) over (= reverse indicator) initially sweeping (initially ‘Sweeping’ = -S) (FLYTRAPS) for the plants open to catching bugs (FLYTRAPS),
2 Eldest in (eldest in) trouble (= anagram indicator) (eldest in = ENLISTED) took a job with a major employer (ENLISTED),
3 Unfinished (y) flat (dreary) (‘dreary’ without ‘y’ = DREAR) is cheerless, as the writers see it (DREAR),
5 Two fruits (cherry + tomato = CHERRY TOMATO) found in salsa (CHERRY TOMATO),
6 Old (O-) Jim (Jim) read (= homophone indicator) (‘Jim’ = ‘gym’ = -PE-) answer (-A-) intercepted by Roger (Roger = -R- GOER) Cook (= anagram indicator) (OPERAGOER), a regular visitor to La Scala (OPERAGOER),
7 Pole (North Pole = N-) is in good time (-EARLY) (NEARLY) or just about (NEARLY),
8 Press a (press a) novel (= anagram indicator) (press a = SPARSE in short supply (SPARSE),
11 Who you might know (SOCIAL CIRCLE) from King Arthur’s confidantes, in a roundabout way presumably (King Arthur’s Round Table = SOCIAL CIRCLE),
15 They’re employed by the Casino (chips = CHIP S-) for spring (-HOP-) season, initially (‘Season’ initially = -S) (CHIP SHOPS) providing takeaways (CHIP SHOPS),
16 One selling Brogan’s (marketing brogans/shoes = SHOE SHOP) topsoil (top ‘Soil’ = S-), garden implements (hoes = -HOE S-) and endless (s) flowers (hops) (‘hops’ without ‘s’ = -HOP) (SHOE SHOP),
17 Miss (-LOSE-) first train (first ‘Train’ = -T-) in Clare (C-E-) going to Dublin (-D) (CLOSETED), as shut up in private meeting (CLOSETED),
19 Lost (= anagram indicator) on Mars (on Mars = RANSOM)? For such a fee, this will get you back! (RANSOM),
20 Function (SOCIAL) opening 11 down (opening ‘social circle’ = SOCIAL),
23 Country (USA-), for example (e.g. = -GE) (USAGE), brings up the matter of consumption (USAGE).