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Thank you to Jason. Definitions are underlined in the clues.
Across
1. PC that is right for item of office kit (6)
COPIER : COP(slang for PC/Police Constable) + IE(abbrev. for “id est”/that is) + R(abbrev. for “right”).
Defn: …, specifically a machine that copies pages by xerography.
4. European Parliament border on time that’s transitory (8)
EPHEMERA : EP(abbrev. for the European Parliament) + HEM(border/edge) plus(on) ERA(a time period).
Defn: Thing ….
10. Knowing about preposterous rubbish in charge for boxing? (7)
STORAGE : SAGE(knowing/wise) containing(about) anagram of(preposterous) ROT(rubbish/nonsense).
Defn: …/cost of storing things, usually packed into boxes, in a warehouse.
11. What makes the room comfortable? Noble narrative (7)
ACCOUNT : AC(abbrev. for “air conditioning”, system that makes the air in a room comfortably cool) + COUNT(a nobleman).
12. Old boring piece for late newspaper article (4)
OBIT : O(abbrev. for “old”) + BIT(a piece for boring holes, as in “a drill bit”).
Defn: An article in a newspaper about someone who recently died/the late someone, giving details of the someone’s life and date of death.
13. This measures angles for tower (10)
PROTRACTOR : PRO-(prefix signifying “for”/in favour of) + TRACTOR(a vehicle for towing, usually found on a farm).
Part of my geometry set in school:

16. Cricket team, say, lodged in hotel eventually (6)
ELEVEN : Hidden in(lodged in) “hotel eventually”.
Defn: A sports team of 11 players, an example of which/say is a cricket team.
17. Rig vote somehow and you’ll get a sickening sensation? (7)
VERTIGO : Anagram of(… somehow) RIG VOTE.
20. Rashly assume Romeo’s chap with healing hands (7)
MASSEUR : Anagram of(Rashly) ASSUME + R(letter represented by “Romeo” in the phonetic alphabet).
21. Singing from the same hymn sheet in concert (6)
UNITED : The two sections of the clue – “Singing … sheet” and “in concert” – mean the same, so not quite a double definition. Which leaves it as a cryptic definition.
24. One cracking reserve vessel in the arctic (10)
ICEBREAKER : Double defn: 1st: A thing that breaks the ice/ends the reserve/inhibitions between people; and 2nd:

25. Face set back with feature of old phones (4)
DIAL : Reversal of(… back) LAID(set/place on). Double defn: 1st: … of a clock or watch, or, informally, a person; and 2nd:

27. Drink with rifleman (7)
MARTINI : Double defn: 1st: A cocktail …; and 2nd: One of the designers of the Martini-Henry rifle.

29. Staff respect changes (7)
SCEPTRE : Anagram of(… changes) RESPECT.
30. Bound by an expression of comfort in little Edward (8)
TETHERED : THERE(or “there, there”, an expression used to comfort/give solace to someone) contained in(in) TED(a diminutive/little of the name Edward).
31. Several hang out on a bright day? (6)
SUNDRY : [SUN-DRY](To dry, say, clothes, by hanging out on a sunny/bright day instead of using the tumble dryer).
Down
1. Who’s always right finds me so curt, sadly (8)
CUSTOMER : Anagram of(…, sadly) ME SO CURT.
Defn: As in the slogan, “The ____ is always right”.
2. In favour of long sentences? Then think highly of spread (11)
PROLIFERATE : PRO-(prefix signifying “in favour of”) + LIFE(long prison sentences/terms) plus(Then) RATE(think highly of/have a high opinion of someone).
3. Being overturned, Mike to dismiss test (4)
EXAM : Reversal of(Being overturned, in a down clue) [ M(letter represented by “Mike” in the phonetic alphabet) + AXE(to dismiss/to terminate the services of) ].
5. I go on quiet train? (8)
PRATTLER : P(abbrev. for “piano”, musical direction to play quietly) + RATTLER(US slang for a fast freight train).
Defn: One who goes on and on talking.
6. Delightful new song, one in English (10)
ENCHANTING : [ N(abbrev. for “new”) + CHANT(a repetitive song) + I(Roman numeral for “one”) ] contained in(in) ENG(abbrev. for “English”).
7. The Muses conceal a fast runner (3)
EMU : Hidden in(… conceal) “The Muses”.
Can’t fly but can run:
8. Plant which flowers close to autumn is behind (6)
ASTERN : ASTER(a flowering plant) + last letter of(close to) “autumn”.
9. Get wind of run in bank (5)
LEARN : R(abbrev. for “run” in cricket scores) contained in(in) LEAN(to bank/to tilt sideways, as with an aircraft making a turn).
14. Miserly drunk clasped duke? (11)
TIGHTFISTED : TIGHT(drunk/intoxicated) + FISTED(clasped/clenched the hand into a fist/duke)
15. Joker with weakness for cups and saucers and the rest (3,7)
TEA SERVICE : TEASER(a joker/one who teases) plus(with) VICE(a weakness/a bad habit).
18. Salutary member of the clergy accommodating drip of a sort (8)
CURATIVE : CURATE(a member of the clergy, assistant to the vicar or parish priest) containing(accommodating) IV(abbrev. for “intravenous”, as in “iv drip”, a medical procedure to introduce fluids into a patient’s bloodstream through a vein).
Defn: … is an archaic term for health-giving.
19. By which real duty gets sullied? (8)
ADULTERY : Anagram of(… gets sullied) REAL DUTY.
Cryptic defn: An act of unfaithfulness which sullies the duty/moral obligation between husband and wife.
22. Smooch being cut short satisfied fate (6)
KISMET : “kiss”(smooch) minus its last letter(being cut short) + MET(satisfied/measured up to).
23. Yard where the sun rises produces fungus (5)
YEAST : Y(abbrev. for “yard”, the unit of length) + EAST(the direction from where the sun rises).
26. Chaps capping upper-class list of options (4)
MENU : MEN(chaps/guys) placed above(capping, in a down clue) U(letter representing things that are upper-class/superior).
28. This deserter upset Jack (3)
RAT : Reversal of(upset, in a down clue) TAR(like “jack”, an informal term for a sailor).
I didn’t think sundry and several meant the same. And I didn’t think much of the clue for UNITED. I didn’t know that MARTINI was a rifleman, but I do now.
I enjoyed this, and was glad of no obscurities or UK-specific references. Thanks, Jason & scchua.
Enjoyed this – as I have all Jason puzzles I’ve come across so far.
I see my own weakness is described in 15d – I do like a nicely presented cuppa! I also ticked TIGHTFISTED, DIAL, STORAGE, TETHERED and especially SCEPTRE.
Plenty of wit on display, as usual.
Thanks to Jason and
Scchua
Excellent puzzle and blog. Still puzzled why fists are dukes. It seems a long stretch from Duke of York via fork to fist.
Thanks to Jason and scchua.
SM@3 To duke it out is to fight it out, and to put up one’s dukes is to put fists up – according to Wiktionary, with various sources, including using dukes in battle.
Fun puzzle, thank you scchua and Jason.
Thanks Shanne. I think it was a challenge used by John Wayne!
Thanks Jason and scchua
SM @ 3 & 5 Per the OED, DUKE meaning hand or fist was first recorded in 1874.
[🙃¡ǝuo pooƃ – ⇂@ɹǝpun uʍop ɟɟoǝƃ]
Thanks Jason and scchua
10ac: I parsed this taking “about” as a reversal indicator for ROT (preposterous rubbish), and then it becomes SAGE [with] TOR in. This avoids needing to assume that an indirect anagram has been used.
21ac: I think this is meant to be two definitions, but I cannot think of it as two meanings.
31ac: The first definition of sundry in Collins 2023 (p 1982) is “several or various; miscellaneous”.
My initial answer to 27a was SHOOTER, both a drink and a rifleman, but I quickly realised its incorrectness when the crossers didn’t work. Once I saw that MARTINI had to be the answer, I eventually looked it up to learn its other possible meaning, as noted above.
An enjoyable, well-clued puzzle. Thanks to Jason and scchua.
Very good, geoff@1.
The comments of GDU@1 and Diane@2 summed it up nicely for me
My favourites were SUNDRY, MASSEUR, PROLIFERATE, and COPIER, but there were plenty of enjoyable clues with good variety
I did scchua’s help to round off my understanding of a couple, so thank you
Thanks Jason and scchua
PB@8 There is a possible further reading of 10ac, if Jason is using the fact that “preposterous” appears as a reversal indicator in Chambers XWD Dictionary: “knowing about” = SAGE, “preposterous rot” = TOR, “in charge” = containment indicator, “for” = link word, “boxing” = answer. I’m not aware of a convincing justification for this use of “preposterous”, as none of the meanings given in Chambers seem to support it, and I note that it does not appear in the list of reversal indicators given by the Clue Clinic.
Rudolf@11: Thanks for that. For preposterous, Chambers 2016 p 1226 has “literally inverted, having or putting the last first” and SOED 2007 p 2331 has “Having last what should be first: inverted”. These are both marked as rare, but not archaic or obsolete. Given that, it seems to me that using “preposterous” as a reversal indicator is no worse than many of the other indicators that we see these days. Then your parsing works, but so does scchua’s if we replace “anagram” by “reversal”.
Looking at its possible derivation, preposterous seems a reasonable reversal indicator. After all, it’s the Latin prefixes for before and after, and seems to indicate that before becomes after.
Exchanging the last and first letters in a word does not necessarily imply that the order of everything in between is also changed. To achieve the desired outcome a reversal indicator needs unequivocally to have the effect of reversing the order of all the letters in the word in question.
Well Preposterous for 10A has to be a reversal indicator rather than an anagram indicator, otherwise we would have a verboten indirect anagram. I had STOWAGE wondering if TOW meant “preposterous rubbish”. Lol. DNK the rifleman. I liked the combo double definition and wordplay for DIAL. LOI PRATTLER took a while to see. Nice one Jason. Thanks to you and scchua.
Rudolf @ 14 You can also flip the components, as per my suggestion @ 13.
LOL, don’t blame RATTLER on us. I’ve lived in the U.S. for 74 years, ridden a lot of trains, and never heard the term rattler.
I’m with Jay on Rattler, though I am hardly a constant rail rider. This was a lot of fun. At first, I was flummoxed by more than half and slowly saw the light. I thought some clues stretched it a bit? For example, I had always heard an anagram clue had to have the actual letters in the clue rather than first taking rubbish to be rot and then making that preposterous. But always learning. Thank you all.
anil@18: The point you made beginning “For example. I had always heard” has been discussed above in comments 8 and 11 to 16 inclusive.