I often struggle with a Filbert Sunday puzzle, and this one certainly had some intricate wordplay; but it fell out nicely in the end.
Abbreviations
cd cryptic definition
dd double definition
cad clue as definition
(xxxx)* anagram
anagrind = anagram indicator
[x] letter(s) removed
definitions are underlined
Across
1 Monsieur enters to have a little soup
CONSOMMÉ
An insertion of M in CON and SOME. The insertion indicator is ‘enters’. ‘I’m sure he’s going to con/have me.’
6 Really enjoy Alien in which ship’s beginning to dissolve with acid
DIGEST
A charade of DIG and an insertion of S for the initial letter of ‘ship’ in ET. The insertion indicator is ‘in which’. The surface is referring to the fact that the first stage of digestion in the gastro-intestinal tract is in the stomach, where the food ingested is broken down with the help of hydrochloric acid.
9 Trying venue where race might be held that accommodates 1,000
COURTHOUSE
An insertion of THOU in COURSE. The insertion indicator is ‘that accommodates’.
10 Anorak‘s over fleece
SCAM
A reversal of MACS. Is a MAC an ‘anorak’?
11 Wii boxing in back of tavern in Thailand, drunken comedy
WITHNAIL AND I
An insertion of N for the final letter of ‘tavern’ in THAILAND, all inserted into WII. The two insertion indicators are ‘boxing in’ and ‘in’ and the ‘drunken’ refers to the film rather than being an anagrind. It was a 1987 comedy which was Richard E Grant’s film debut and featured a lot of scenes in the pub.
13 Sponge without water perhaps upsetting Mary, die on cross
READY-MIX
(MARY DIE)* plus X. The anagrind is ‘upsetting’. The READY-MIX is the cake variety but the surface is alluding to the offer of a sponge soaked in vinegar to Christ on the cross and the fact that John’s Gospel records Mary standing near the cross.
And one ran and filled a spunge full of vinegar, and put it on a reed, and gave him to drink [Mark 15.36, KJV]
15 Stick one’s china back on mantelpiece
IMPALE
A charade of I’M, PAL and E for the final letter of ‘mantelpiece’. China is cockney rhyming slang: china plate = mate = pal.
16 Japanese island ducks largely avoid
HONSHU
A charade of HON and SHU[N]. HON and ‘ducks’ are both terms of endearment: HON is short for ‘honey’ and ‘ducks’ is regional dialect.
18 Accurately position leg and off-side fielder
PINPOINT
A charade of PIN and POINT. Where would crossies be without cricket?
20 Old record tied yet broken in prospect
SEVENTY-EIGHT
An insertion of EVEN and (YET)* in SIGHT. The insertion indicator is ‘in’ and the anagrind is ‘broken’.
23 Beat little pest that won’t let go
TICK
A dd.
24 Decent plastic ID bracelet
CREDITABLE
(ID BRACELET)* with ‘plastic’ as the anagrind.
25 Bill right to replace note in coins
CHARGE
Filbert is inviting you to replace the N in CHANGE with R.
26 Consider change to clue, perhaps Mike worried about it?
MEDITATE
An insertion of EDIT in M and ATE. The insertion indicator is ‘about it’.
Down
2 Bit of wind taking the wings off two Boeings
OBOE
Hidden in twO BOEings.
3 Son worked on boat up the creek
SCREWED
A charade of S and CREWED.
4 Rug on top for Spooner? Surely not
MY HAT
A Spoonerism of HIGH MAT.
5 Shortened horsy draught animal’s 12 hour day
EQUINOX
A charade of EQUIN[E] and OX.
6 Scoffing expert finally said “That is an old master”
DIETITIAN
A charade of D for the final letter of ‘said’, IE for id est and TITIAN.
7 Burning light alarmingly close in space
GAS LAMP
An insertion of SLAM in GAP. The insertion indicator is ‘in’.
8 Acting independently troubled Nelson a tad
STAND-ALONE
(NELSON A TAD)* with ‘troubled’ as the anagrind.
12 Get rid of husband, but shag single comedian first
BE DONE WITH
A charade of BED, ONE, WIT and H.
14 You must cut extremely long hair
MOUSTACHE
An insertion of U (textspeak for ‘you’) in MOST followed by ACHE. The insertion indicator is ‘must cut’.
17 Shoe reeks, case of aeration failing?
SNEAKER
(REEKS A[ERATIO]N)* with ‘failing’as the anagrind.
18 Half retire in style earlier than expected
PRETERM
An insertion of RET[IRE] in PERM. The insertion indicator is ‘in’.
19 How to serve cheese and beans? Not with head inside oven
ON TOAST
A charade of ONT and OAST. The instructions tell you to put the first letter, or ‘head’ of NOT in the centre of the word.
21 Put precious coat on to cover uniform for society
GUILD
An insertion of U for the phonetic alphabet ‘uniform’ in GILD. The insertion indicator is ‘to cover’.
22 Slow mover missing the last scheduled time
SLOT
SLOT[H]
Many thanks to Filbert for this Sunday’s puzzle.

Agree with your introduction.
Although I originally parsed OBOE as you, Pierre, I prefer twOBOEings without t’ wings.
You are right, Hovis. It is hidden, but in the way you point out – thank you.
Parsed OBOE as Hovis. Maybe the setter could have alluded to Yorkshire to give us the shortened ‘the’ = T – though that is the kind of thing that might appear in the OED. It did not seem like Filbert to have uneven wings to remove from the fodder. I was pleased to remember HONSHU but did not parse that one.
Thanks both
WITHNAIL AND I definitely in my top five movies of all time. I can remember laughing hysterically throughout when I saw it in Paddington in Sydney in the 1980s. Went in quick. Elsewhere I enjoyed ‘slam’ for ‘alarmingly shut’ in GASLAMP and the clue and the blog’s backstory for READY MIX. Such a great yarn and citation. Makes me wish I was religious. But I’m not. Appreciate the blog Pierre and thanks Filbert.