Solution to 21A 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.
Open for anyone to answer, if not solved by 1 PM.
ACROSS
1 Masked traffickers and escorts (6) USHERS pUSHERS
5 Dismiss exhibitionist replacing old woman with runner (5,3) SHRUG OFF {SH(-ow+rug)RUG OFF}
9 Tackle a very soft insect (8) APPROACH {A}{PP}{ROACH}
10 I study a great deal of Frost...Classic! (6 ) ICONIC {I}{CON}{ICe}
11 Control panels? So drab! (10) DASHBOARDS {SO+BRAD}* [RA]
13 Uzbekistani Techie Lobo Agnus seen regularly in winter sport (8) BIATHLON {uzBekIstAni+TecHie+LobO+agNus}
17 Recoiled after Superintendent of Police gave a buzz (6) SPRANG {SP}{RANG}
19 The Don is thus sheltering a pleasure seeker (8) HEDONIST* [T]
21 Box picked up for a piece of metal at end of an arrow (4) ?O?K (Addendum NOCK (~knock) See comments)
25 Indian state, large and mountainous (6) UPLAND {UP}{L}{AND}
26 Narcissistic outings? (3,5) EGO TRIPS [CD]
27 Nonsense crew, declares Spooner - withdraw! (4,4) PULL BACK (~bull pack to pull back)
DOWN
2 Subtle, earthy pigmentation in art-galleries basically? (5) SEPIA Acrostic &lit
3 Elements of objective art highlighting our planet (5) EARTH [T]
4 Look legendary Welsh King delivered finely ground pulverised material (3,4) SEACOAL (~see cole)
5 Asha somehow managed one originating from the African Desert (7) SAHARAN {ASHA}*{RAN}
6 Rise up and plead for release once more (7) REISSUE {RISE*}{SUE}
7 Goths won't bust a deserted community (5,4) GHOST TOWN*
8 I sent fans out in exhaustion (9) FAINTNESS*
14 Unprepared, independent member of parliament meets Trump casually around 1st of October (9) IMPROMPTU {I}{MP}{R{Oc...r}MPTU*}
15 Giant bearing pain with black electronic device (9) TRACKBALL {T{RACK}{B}ALL}
18 Ultimately amazing supporter, a superman, proud champ 'Dada' (7) GRANDPA [Tail Acrostic] Semi&lit
20 Are these drug dealers making a mess of Taco orders? (7) DOCTORS {DOCTORS+ARE}*=TACO ORDERS
23 Tarnished parts of spiky trident held up (5) DIRTY [T<=]
24 The French writer sent back to country (5) NEPAL {LA}{PEN}<=
Reference List
Old = O, Woman = W, Very soft = PP, Study = CON, Journo = ED(itor), Superintendent of Police = SP, Large = L, Independent = I, Member of Parliament = MP, Black = B, Kevin = K, The in French = LA
