Solution to 13A 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 Turning excited on cocaine, gets into fight in bar tour (3,5) PUB CRAWL {UP<=}{B{C}RAWL}
5 Loads volcanic rock aboard steamer (6) STUFFS {S{TUFF}S}
10 Musical instrument in case picked up by versatile ace (7) CELESTA {CE{LEST}A*}
11 Sea cucumber’s flavour enthrals agent (7) TREPANG {T{REP}ANG}
13 Trouble all round after associate faces depression temporarily (2,7) A? I?T?R?M (Addendum - AD INTERIM {MIRE<=} after {A}{DINT} - See comments)
14 On beer and pot, let loose — it’s what drinker needs (6-6) BOTTLE-OPENER {ON+BEER+POT+LET}*
18 Knocking back liquor on the house, boors go berserk in American city (12) MURFREESBORO {RUM<=}{FREE}{BOORS*}
21 Furnace and boiler boiling over! Run away (3,6) OIL BURNER {BOILER}* over {RUN}*
23 Asian city overwhelmed by outbreak as overcrowding returns (5) OSAKA [T<=]
25 How a Conservative MP might describe self as romantic (7) AMATORY {AM A TORY}
26 Body found in restroom at terminus (6) MATTER [T]
27 Suspicious cartons contain drug precursor (8) ANCESTOR {ANC{E}STOR*}
DOWN
1 Quick to embrace single female and soothe (6) PACIFY {PAC{1}{F}Y}
2 Cry of pain after charges surge (6) BILLOW {OW}<=>{BILL}
3 River or sea perhaps, not a lake (9) RESERVOIR {RIVER+OR+SEa}*
4 Overcome by strong emotion, like a person with osteoarthritis might be? (4,2,3,5) WEAK AT THE KNEES [DD]
6 Greek character almost catches fellow shoplifting (5) THEFT {THE{F}Ta}
7 Notorious ex-convict managed to enter empty flat (8) FLAGRANT {Fla{LAG}{RAN}T}
8 Native American chief upset about reactionary slogan used by Trump (8) SAGAMORE {S{MAGA<=}ORE}
9 Make a deal with effort, acquiring a furniture company by take-over (6,1,7) STRIKE A BARGAIN {STR{IKEA}{GRAB<=}AIN}
16 Unusually mobile, small mass causes obstruction of blood vessel (8) EMBOLISM {MOBILE*}{S}{M}
17 Amateur correctly grips both sides of coil in device providing illumination (8) ARCLIGHT {A}{R{CoiL}IGHT}
19 Sozzled actor popping bit of Viagra in romp (6) CAVORT {CA{Vi...a}ORT*}
20 Brief film about onset of war (6) LAWYER {LA{War}YER}
22 Bones in middle of trunk alongside seedy lane (5) ULNAE {trUnk}{LANE*}
Reference List
Cocaine = C, Steamer = SS, Fine = F, Associate = A, Can = LOO, Boy = B, Drug = E, Female = F, Fellow = F, Small = S, Mass = M, Amateur = A



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