Thanks Falcon for an enjoyable puzzle. Definitions are underlined in the clues.
It’s another pangram.
Across
1 Pack case for accordion (7-3)
SQUEEZE BOX : PACK(to cram/squeeze into) + BOX(a case for items such as pencils, say).

7 Shrewd signal saving time (4)
CUTE : CUE(a signal to someone to start, say, speaking) containing(saving) T(abbrev. for “time”).
Defn: …/cunning, as in the admonishment: “Don’t be cute with me!”
9 Jack plug taken by English hack (4)
JADE : J(abbrev. for “Jack”, as in the Jack of, say, Hearts playing card) + AD(short for “advertisement”/a plug/a promotional item) plus(taken by) E(abbrev. for “English”).
Defn: …/a worn-out horse.
10 No lines on me in FT, say, after I invested in scheme (5,5)
PLAIN PAPER : PAPER(an example of which/say, is the Financial Times/FT) placed after(after) [I contained in(invested in) PLAN(a scheme/a plot)].
11 Cast figure, standing, not right (6)
STATUE : “stature”(one’s standing/importance) minus(not) “r”(abbrev. for “right”).
12 Prayed uneasily in heart of Tashkent and here, in London (4,4)
HYDE PARK : Anagram of(… uneasily) PRAYED contained in(in) middle 2 letters of(heart of) “Tashkent“.
13 I met GP for treatment involving one old skin complaint (8)
IMPETIGO : Anagram of(… for treatment) [I MET GP plus(involving) I(Roman numeral for “one”) + O(abbrev. for “old”)].
15 Doctor holding a male’s umbrella (4)
GAMP : GP(abbrev. for “general practitioner”/a doctor) containing(holding) [A + M(abbrev. for “male”)].

17 Unsatisfactory, a king of Mercia (4)
OFFA : OFF(unsatisfactory/unacceptable) + A.
Defn: … from 757 to 796.
19 Passenger ship that is stocking good underwear (8)
LINGERIE : LINER(a passenger leisure ship) + IE(abbrev. for “id est”/that is) containing(stocking) G(abbrev. for “good”).
22 Sitar playing – hear genius (8)
ARTISTRY : Anagram of(… playing) SITAR + TRY(to hear in a court of law).
23 WI member appearing with the French nun’s headdress (6)
WIMPLE : WI + MP(abbrev. for a Member of Parliament) plus(appearing with) LE(French for “the”).

25 Book produced by Italian in navy, awfully objective (6,4)
VANITY FAIR : IT(abbrev. for “Italian”) contained in(in) anagram of(…, awfully) NAVY + FAIR(objective/unbiased).

26 Very large jewel brought over by American (4)
MEGA : Reversal of(… brought over) GEM(a jewel/a precious stone) plus(by) A(abbrev. for “American”).
Defn: Prefix for a very large thing.
27 Moving after I lost medal (4)
GONG : “going”(moving/travelling) minus(after … lost) “I“.
28 Relish last of salad? Unfortunate dilemma (4,6)
DILL PICKLE : Last letter of(last of) “salad” + ILL(unfortunate/unfavourable, as in “a run of ill-luck”) + PICKLE(a dilemma/a difficult situation in which one finds oneself).
Down
2 Question abnormal autumn amount (7)
QUANTUM : Q(abbrev. for “question”) + anagram of(abnormal) AUTUMN.
3 Incident in flat ending in arrest (5)
EVENT : EVEN(flat/smooth, as in “an even surface”) + last letter of(ending in) “arrest“.
4 One of Groucho’s brothers almost landed on river, going north in airship (8)
ZEPPELIN : “Zeppo”(youngest of Groucho Marx’s brothers) minus its last letter(almost) placed above(landed on, in a down clue) reversal of(…, going north, in a down clue) NILE(Egyptian river).
5 Have bellboy call, different game being required (5,10)
BEACH VOLLEYBALL : Anagram of(…, different) HAVE BELLBOY CALL.

6 Kane’s estate secured by Texan, a dude (6)
XANADU : Hidden in(secured by) “Texan, a dude“.
Defn: The fictional estate of Charles Foster Kane in Orson Welles’ film “Citizen Kane”.
7 Winner, Ms Grey, mostly bubbly? (9)
CHAMPAGNE : CHAMP(champion/winner) + “Agnes”(Grey, the governess in Anne Bronte’s book) minus its last letter(mostly).
8 Proposition made by Greek boy band? (7)
THEOREM : THEO(a boy’s name/a diminutive of masculine names, such as Theodore, all of which are of Greek origin) + R.E.M.(an American rock band in the 80s).
14 Questioning former Ugandan dictator in Ghana’s capital (9)
EXAMINING : EX-(prefix meaning “former”/once) + AMIN(Idi, former Ugandan dictator) + IN +1st letter of(…’s capital) “Ghana“.
16 Flower at present lying in small ditch (8)
SNOWDROP : NOW(at present/as of this moment) contained in(lying in) [S(abbrev. for “small”) + DROP(to ditch/to dump)].

18 Hotchpotch in Portuguese resort shown round daily, perhaps (7)
FARRAGO : FARO(resort town in Portugal) containing(shown round) RAG(a newspaper/a daily regarded as being/perhaps of low quality).
Defn: …/a mishmash.
20 Bad carrying on with a criminal (7)
ILLEGAL : ILL(bad/unfortunate, as in “ill-fortune” – cf. 28 across) containing(carrying) [LEG(on/the half of the cricket field behind the batsman as he stands to receive the ball) plus(with) A].
21 Remained sober, by the sound of it (6)
STAYED : Homophone of(…, by the sound of it) “staid”(sober/respectable).
24 Parody of opera heroine, cold (5)
MIMIC : MIMI(heroine of Puccini’s opera, La Boheme) + C(abbrev. for “cold”).
Thanks, scchua. My LOI was JADE, which I wouldn’t have solved if I hadn’t spotted the pangram. It was a gentle, well-constructed puzzle that brought great pleasure to an early breakfast; it was impossible to feel jaded. Thank you, Falcon.
Exactly the same comments re JADE as Hornbeam @1. I took as long on this innocent looking clue as I did on the rest of puzzle.
Apart from this bit of frustration at the end, a very pleasant way to fill in the time. Favourite was WIMPLE, one of those words I just like the sound of and because it reminds me of Audrey Hepburn in “The Nun’s Story”.
Thanks to Falcon and scchua
Thanks to Falcon and scchua. Very enjoyable. I did not have trouble with JADE but GAMP-umbrella was new to me and I keep forgetting that GONG = medal-award (though both were clearly clued).
Enjoyable but a Mondayish puzzle. LOI JADE.
Thanks Falcon and scchua (for the usual comprehensive and illustrative blog)
A typically quicker solve than the average FT puzzle, although the pangram was unexpected and there was a little more general knowledge required than normal. All clues as usual, meticulously fair.
Getting SQUEEZE-BOX quickly followed by JADE raised the possibility of a pangram straight away although it was not required to finish off any last tricky clues.
Finished in the SE corner with MIMIC, DILL PICKLE and MEGA the last few in.