Solution to ??? 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 Thinking about article on origin of Kazakh people (7) MANKIND {M{AN}{Ka...k}IND}
5 Scrap union's rule amidst return of Oriental pandemic disease (7) DIVORCE {E}C{R}OVID}<=
9 Building in college to preserve antique receptacle for clothing (7) HOLDALL {H{OLD}ALL}
10 Consumer of weekend US erotica (3-4) END-USER [T]
16 Agave fibre from Lemhi Pass in a pickle (5,4) SISAL HEMP*
18 Country, say, Vespucci summarised incompletely in review (5) MUSIC [T<=]
21 Grand Ormolu that stands for path of integrity? (4,5,6) HIGH MORAL GROUND {GRAND ORMOLU}* [RA]
26 Provided drink after unwrapping wine (7) OFFERED {cOFFEe}{RED}
27 Holding rings for miniature speakers, they play the horn (7) TOOTERS T(+oo-wee)OOTERS
DOWN
1 Old-time New Yorkers' way of working with birds (7) MOHAWKS {MO}{HAWKS}
4 Boy climbing atop villa overlooking one Himalayan town (9) DALHOUSIE {LAD<=}{HOUS{1}E}
5 Love holding king down (5) DREAR {D{R}EAR}
7 Ship arose eerily outside the Last Ocean (4,3) ROSS SEA {SS} in {AROSE}* Can a sea be called an ocean?
8 Order first item from rose new incense boxes (7) ENRANGE {ENRA{New}GE} - Confirmed by Avtaar. ROSE being changed to NEW in the online version.
13 Person lighting weak explosive drilling under hollow (9) ILLUMINER {ILL}{Und{MINE}eR}
15 One taking fight to the next round? (9) APPELLANT [CD]
17 Stop working of fins moving around cap in generator (4,3) SIGN OFF {OF+FINS}* over {Ge...r}
19 Fall way behind after cycling ends in awful failure (7) STUMBLE (+s)STUMBLE(-s) Not sure of Anno
20 Beggars start to dance in prison with discharged rangers (7) CADGERS {CA{Da..e}GE}{Ra..eS}
22 Searched thoroughly, runs with nude shedding top (5) RAKED {R}{nAKED}
23 Old brown liquor lacking pressure sent up (5) RETRO pORTER<=
Reference List
Rule = R, Oriental = E, International = I, Popular = IN, Drug = E, Way of working = MO(Modus Operandi), King = R, Affair = DO, Earl = E, Ship = SS, Runs = R, Pressure = P