No 13728, Monday 05 Dec 2022, Arden

Solution to 20D 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   Top of ground is wet, surface is rough (6) GRAINY {Gr...d}{RAINY}

4   Charlie's voice of indignation at Chinese food (4,4) CHOW MEIN {C}{(~how mean)HOW MEIN}

9   Calling it neat - oddly it's China (6) NAMING {NeAt}{MING}

10 "Corrupt from day one" - recalled indent (8) DECADENT {D}{ACE}<= in {DENT}

12 Liquid Sodium carbonate not counted as an elixir (8) AMBROSIA {SOdIuM+cARBonAte}*

13 Partly release the Neon gas (6) ETHENE [T]

15 Service built is how cars are manufactured today (4-8) MASS-PRODUCED {MASS}{PRODUCED}

18 Movie online - it could affect your mood (12) PSYCHOTROPIC {PSYCHO}{TROPIC}

21 Snake has taken shelter, is resting (6) ASLEEP {AS{LEE}P}

22 Old Italian cavorted with centaurs (8) ETRUSCAN*

24 Left wingers with shoddy clothes accept right winger (8) REDSTART {REDS}{TA{R}T}

25 Essentially sent to check weapon - it's a threat (6) MENACE {M{sENt}ACE}

26 Agreed to arrange outboard sporting event (8) DRESSAGE {DRE{SS}AGE*}

27 Usher first customer into fancy store (6) ESCORT {ES{Cu...r}ORT*}

DOWN

1   By midnight, Arden becomes Arden the policeman (8) GENDARME {niGht}{ARDEN*}{ME}

2   Perhaps Mary found a military establishment (4,4) ARMY BASE {MARY*}{BASE}

3   Used mnemonics - spot on when one can't think clearly (3,6,6) NON COMPOS MENTIS MNEMONICS+SPOT+ON}*

5   List to follow (4) HEEL [DD]

6   Advice to shun dhoti if one wants to be assertive at home (4,3,8) WEAR THE TROUSERS [C&DD]

7   Opening allowed under watch (6) EYELET {LET}<=>{EYE}

8   Spice girl goes under - crazy! (6) NUTMEG {MEG}<=>{NUT}

11 Teacher loves to cover two chapters - it's just hot air (7) SIROCCO {SIR}{O}{CC}{O}

14 Group wearing pretty split skirt (7) CULOTTE {CU{LOT}TE} 

16 A technique in playing violin, about to be covered with various topics (8) SPICCATO {SPIC{CA}TO*}

17 Little houses, study coming up (8) SCANDENT {SCAN{DEN}T}

19 Crazy to be supported by free capital (6) MADRID {MAD}{RID}

20 Security is prim if taken separately (6) ?L?D?E (Addendum - PLEDGE {P}{LEDGE} - See comments) 

23 Dress up and boast (4) BRAG<=

Reference List

Charlie = C, Day = D, Shelter = LEE, Shoddy = TAT, Right = R, Chapter = C, About = CA