Solution to 18A has been deliberately left unsolved and is to be answered only by a non-regular / novice commenter, with proper annotation. Those who have answered earlier in the week, please give others a chance.
ACROSS
1 Drunkard drinking 'The Macallan'! At heart is a highlander! (4) SCOT {S{T...mCa...n}OT}
4 It's amateur kind of distress (10) TRAUMATISE*
9 Cooking idli! Guy's hard-working! (8) DILIGENT {IDLI}*{GENT}
10 Online shopping place - a mother's territory mostly (6) AMAZON {A}{MA}{ZONe}
11 Tie? Large one! (4) LACE {L}{ACE}
12 Popular English feature - 'Prejudice' (10) INEQUALITY {IN}{E}{QUALITY}
13 Bachelor routine, almost completely beastly! (6) BRUTAL {B}{RUT}{ALl}
15 A microscopic part like pupil (8) EYEPIECE {EYE PIECE}
17 Victory ends before time for the most bombastic (8) WINDIEST {WIN}{DIES}{T}
18 Bid to make bet? Trade thus in beef! (6) E?F?R? (Addendum - EFFORT {BET with (EF FOR T) becomes BEEF}
19 Old cribs! Set to do childbirth management! (10) OBSTETRICS {O}{CRIBS+SET+TO}* There's an extra O in the fodder
21 Open, mutinous refusal (4) NOPE*
23 Immediately in favour of not working (6) PRONTO {PRO}{NOT*}
24 Jewish priest embracing US President, retrospectively looks distasteful! (8) INEDIBLE {EL{BIDEN}I}<=
25 Outdated fellow raves about English cosmetics (10) DEODORANTS {D{E}ODO}{RANTS}
26 Royal, mostly ahead of time (4) EARL EARLy
DOWN
2 Friend from church in America (5) CHINA [T] Rhyming slang
3 Experimented with clothes! Party is split into three parts (9) TRISECTED {TRI{SECT}ED}
4 Neighbourhood Jane? (3,4,4,4) THE GIRL NEXT DOOR [CD]
5 Moving out in a car, one going around to pick a professional who repairs car batteries say (4-11) AUTO-ELECTRICIAN {OUT+IN+A+CAR+1}* over {ELECT}
6 Male user senses deceptively small unfathomable quality (15) MEASURELESSNESS {MALE+USER+SENSES}*{S}
7 Dog catching right scent (5) TRAIL {T{R}AIL}
8 Flaunt injury mark? 911 on wheels, say! (6-3) SPORTS-CAR {SPORT}{SCAR}
14 Right! That is about men at stock exchange giving out zero refund! (9) REIMBURSE {R}{IE<=}{M}{BoURSE}
16 Popular line about hill dweller is puerile (9) INFANTILE {IN}{F{ANT}ILE}
20 United without unity! Somehow brought to harmony! (5) TUNED UNiTED*
22 Love left trapped by mean cold (5) POLAR {P{O}{L}AR}
Reference List
Large = L, Popular = IN, English = E, Bachelor = B, Time = T, Old = O, Small = S, Right = R, Men = M, Left = L