Thank you to Poins. Definitions are underlined in the clues.
Across
1. Soldier and sailor returning after leave (6,3)
DESERT RAT : Reversal of(… returning) TAR(informal term for a sailor) placed after(after) DESERT(to leave/abandon).
Defn: … in the British Army armoured division that fought in the Egyptian and Libyan deserts in WWII.
6. Die for soft rugs from the east (5)
STAMP : Reversal of(… from the east) [P(abbrev. for “piano”, musical direction to play softly) + MATS(rugs/pieces of coarse material laid on the floor)].
9. Girl maintaining it’s indispensable (5)
VITAL : VAL(short for Valerie, a girl’s name) containing(maintaining) IT.
10. Breast feeding Polish queen becoming abrasive perhaps (9)
SANDPAPER : PAP(a woman’s breast or nipple) contained in(feeding) [SAND(to polish/smoothen with, well, sand paper, say) + ER(abbrev. for “Elizabeth Regina”, the Queen)].
11. Handy to have tin for cash (5,5)
READY MONEY : READY(handy/easily available) plus(to have) TIN(slang for money).
12. Suffer when exposed in speech (4)
BEAR : Homophone of(… in speech) “bare”(exposed/with no covering).
14. Poins is taken in by girl’s facial expression of disgust (7)
GRIMACE : I’M(“I am”/”Poins is”, with the setter using the self-referential pronoun) contained in(taken in by) GRACE(a girl’s name).
15. Went to protect my back in advance (7)
PROCEED : PEED(went/unrinated) containing(to protect) reversal of(… back) COR!(like “my!”, an expression of surprise).
17. Tolerant fellow and striking girl doing a bunk (7)
LENIENT : LEN(a fellow/chap’s name) + “salient”(striking/most noticeable) minus(… doing a bunk) “Sal”(a girl’s name).
19. Artist‘s mother to pose for painters around the London area (7)
MATISSE : MA(informal term for one’s mother) + reversal of(… around) SIT(to pose for painters/artists) + SE(abbrev. for “southeast”, the area of England in which London is; alternatively, “South East”, the postcode area in London).

20. Supporter finding witty remark after Sunak’s first to resign (4)
ALLY : “sally”(a witty remark in response) minus 1st letter of(after …’s first to resign) “Sunak’“.
22. Exceptional talent shown by Bill in race abroad (10)
BRILLIANCE : Anagram of(… abroad) BILL IN RACE.
25. Happen to be right about anger following answer by Penny (9)
TRANSPIRE : Reversal of(… about) RT(abbrev. for “right”) + {IRE(anger/rage) placed after(following ) [ANS(abbrev. for “answer”) plus(by) P(abbrev. for “penny”, the unit of British currency)]}.
26. What Madame Defarge does fools king in the beginning (5)
KNITS : NITS(fools/idiots) placed after(… in the beginning) K(abbrev. for “king”, in chess notation).
Defn: … in Charles Dickens’ A Tale of Two Cities.
27. Grant makes a profit (5)
YIELD : Double defn: 1st: …/concede a point of dispute, as in “I grant that he did do a good job”; and 2nd: …/a financial return.
28. Officer extremely lucky as a rule (9)
GENERALLY : GENERAL(a high-up officer in the military) + 1st and last letters of(extremely) “lucky“.
Down
1. Daughter having too much port (5)
DOVER : D(abbrev. for “daughter”) plus(having) OVER(too much/more than).
Defn: … on England’s south coast.
2. Office suit annoyed by legal proceedings about to be dismissed (9)
SITUATION : Anagram of(… annoyed) SUIT plus(by) “action”(legal proceedings/a lawsuit) minus(… to be dismissed) “c”(abbrev. for “circa”/about when refering to years or other periods of time).
Defn: A position of authority/employment.
3. Rarely messed up by experts in team events (5,5)
RELAY RACES : Anagram of(… messed up) RARELY plus(by) ACES(experts/those who excel).
4. Do up shop near the centre of Alfreton (7)
RESTORE : STORE(a shop/a retail outlet) placed below(near, in a down clue) 2 middle letters of(centre of) “Alfreton“.
5. Worried teens in court become unable to relax (5,2)
TENSE UP : Anagram of(Worried) TEENS + UP(in a court of law/appearing for trial before a judge or magistrate, say).
6. Numberless photos of foolish people (4)
SAPS : “n”(symbol for “number”, in mathematics) deleted from(…less) “snaps”(photos/snapshots).
7. Specimen not originally large enough (5)
AMPLE : “sample”(a specimen/a small quantity taken from a whole for analysis) minus its 1st letter(not originally).
8. Game failing to start after concern for winger possibly (9)
PARTRIDGE : “bridge”(a card game) minus its 1st letter(failing to start) placed below(after, in a down clue) PART(concern for/responsibility towards, as in “I want no part of that scheme”).
Defn: An example/possibly of a creature with wings.
13. Head of languages getting extremely upset during squabble with servile flatterer (10)
BOOTLICKER : [1st letter of(Head of) “languages” placed below(getting, in a down clue) reversal of(… upset) TOO(extremely/very)] contained in(during) BICKER(to squabble/to argue about petty and trivial matters).
14. Bravery of girl soldier captured in Lowell’s opening lines (9)
GALLANTRY : GAL(girl) + ANT(a caste of whose insect society is the soldier) contained in(captured in) [1st letter of(…’s opening) “Lowell” + RY(abbrev. for “railway”/system of train lines/tracks)].
16. In distress it’s seen by almost everybody as absolutely necessary (9)
ESSENTIAL : Anagram of(In distress) IT’S SEEN plus(by) “all”(everybody) minus its last letter(almost …).
18. Rotation is good after shock at home (7)
TURNING : G(abbrev. for “good”) placed below(after, in a down clue) [TURN(a shock/a start, as in “that sudden noise gave me quite a turn”) + IN(at home/not out of one’s house)].
19. Ponder over trace of ergot on popular plant (7)
MULLEIN : MULL(to ponder/to think about deeply and at length) placed above(over, in a down clue) 1st letter of(trace of) “ergot“placed above(on, in a down clue) IN(popular/in fashion).

21. Let loose at the outset then relax (5)
LEASE : 1st letter of(… at the outset) “loose” plus(then) EASE(to relax/to make less severe).
Defn: … a property, say.
23. Attempt to speak in support of drugs (5)
ESSAY : SAY(to speak/verbalise) placed below(in support of, in a down clue) ES(plural of E/abbrev. for the drug Ecstasy).
24. Employed by some pious educators (4)
USED : Hidden in(some) “pious educators“.
Are we there yet? Yes, or solved anyway.
Good to have one that didn’t involve too much hair-pulling, but even so I couldn’t parse the ‘striking girl doing a bunk’ bit of LENIENT and I didn’t know PAP for ‘Breast’. I was also put off by ‘winger possibly’, rather than just ‘winger’, for PARTRIDGE. Doing a search post-solve, I found a footballer called Scott Partridge (never heard of him before), who played as a winger for at least some of his professional career during the 1990’s-2000’s and I wonder if Poins may have been referring to him.
Thanks to scchua (including for the pic of the didn’t-have-a-clue-what-it-looked-like MULLEIN) and to Poins for a pleasant Sunday am solve
Like WordPlodder I wondered why ‘possibly’ was included at 8D; the footballing winger is hardly likely to come to mind (he didn’t to mine) as an alternative, but does it matter? The most famous paps are surely those of Jura, the twin breast-shaped summits of that beautiful Hebridean island, though from some angles there are three! Thanks Poins and Sschua.
No bickering from me this morning after a steady solve and parse with BOOTLICKER holding out til last as I had a fixation on the L coming first for a while. I was also guilty of carelessness with 3d thinking “Poins hasn’t given us the right fodder for RALLY RACES” before looking again more carefully. PARTRIDGE is slightly odd, I agree. Funnily enough, I was helped there by it being a game bird – though I appreciate game wasn’t being used in that way. PROCEED, LENIENT, SITUATION and ESSENTIAL are my pick of the bunch today.
Thanks Poins and scchua (Gosh – spellchecker does NOT like that sign off!)
That brought back happy memories of wild mulleins in the Bulgarian hills. All good accessible fun. Alan Partridge as one who wings it????
All done and dusted with no problems except that it was over too quickly. Favourite was 19ac with “Artist’s mother” suggesting a different artist.
Thanks, Poins and scchua.