Solution to 16D has been deliberately left unsolved and is to be answered by a non-regular/novice commenter, with proper annotation. Those who have answered earlier in the week, please give others a chance.
Open for anyone to answer, if not solved by 1 PM.
ACROSS
1 Humorist and married assistant in retreat, cheating around (8) COMEDIAN {CO{M}{AIDE<=}N}
5 Quarrel about American beginning to raise tax (6) TARIFF {T{A}{Ra..e}IFF}
11 Arrive in market to get hold of vase (4,3) TURN OUT {T{URN}OUT}
12 Member, upset after trustee's last electronic message (1-4) E-MAIL {M}{AIL}<=>{t...eE}
13 Developing nice grin perhaps to capture hearts (9) ENRICHING {NICE+GRIN}* over {H}
14 One seen at racecourse painting popular horse during ceremony (8,4) STARTING GATE {ST{ART}{IN}{GG}ATE}
18 What person might do in cold weather to show keen satisfaction (3,4,5) RUB ONES HANDS [DD]
21 Immorality of English rogue enthralled by joint and cocaine on vacation (9) DECADENCE {D{E}{CAD}EN}{Co...nE}
23 Obstructive problem in part of small intestine, nearly ending in peritonitis (5) ILEUS {ILEUm}{p...iS} Semi&lit
24 Crowd heartily entertained by genius playing type of rock (7) IGNEOUS {IGNE{crOwd}US*}
25 Crack cocaine - extremely repulsive bad habit (7) CREVICE {C}{Re...vE}{VICE}
DOWN
2 Being ethical, admits fault finally (6) MORTAL {MOR{f..lT}AL}
3 Misleads essentially in uncertainty, playing only two cards of a suit (9) DOUBLETON {DOUB{misLEads}T}{ON}
4 Incompetence of masseur in the spa, extremely clumsy (14) AMATEURISHNESS {MASSEUR+IN+THE+SpA}*
6 A posh valuable, mostly of gold (5) AURIC {A}{U}{RICh}
7 Comic in Cairo, ultimately satirical and wry (8) IRONICAL {IN+CAIRO}*{s...aL}
8 Overweight guide, terribly exhausted (8) FATIGUED {FAT}{GUIDE*}
9 Trump is beginning to argue at once, making fuss about trivial matter (5,2,1,6) STORM IN A TEACUP {TRUMP+IS+Ar..e+AT+ONCE}*
15 Singles romping, having party in retreat? Goodness! (9) GODLINESS {G{DO<=}LINESS*}
16 Resenting misuse of drug and drink by girl (8) ?R?D?I?G ()
17 Old boy on screen, turning more indecent (8) OBSECNER {OB}{SCREEN*}
19 Crew in department checking VCR button (6) REWIND [T]
20 They question fool grabbing king and queen (6) ASKERS {AS{K}{ER}S}
22 Decided to acquire manufacturer's latest unmanned aerial vehicle (5) DRONE {D{m...eR}ONE}
Reference List
Married = M, Member = M, Hearts = H, Popular = IN, Horse = GG, English = E, Cocaine = C, Caught = C, Posh = U, Party = DO, Old Boy = OB, King = K, Queen = ER
