Thank you to Harpo. Definitions are underlined in the clues.
Across
4. Fuss follows Kipling novel about Japanese emperor (6)
MIKADO : ADO(fuss/commotion) placed after(follows) reversal of(… about) KIM(novel by Rudyard Kipling).
Defn: An archaic term for the ….
6. Eg x, y or z having a range of possible values (8)
VARIABLE : In mathematics, a symbol for a number that can change/have a range of values, eg. x, y, or z.
Not cryptic?
9. Snack wherein raspberries somewhat making a comeback (6)
SARNIE : Hidden in(… somewhat) reversal of(… making a comeback) “wherein raspberries”.

10. Angrily attacks plump child (6,2)
ROUNDS ON : ROUND(well-filled out/plump) + SON(a familiar term of address for a child).
11. Awful magnate uses relief (11)
ASSUAGEMENT : Anagram of(Awful) MAGNATE USES.
15. Outdo Kew Gardens, ultimately plugging trim variety of primula (7)
COWSLIP : Last letters, respectively, of(…, ultimately) “Outdo Kew Gardens” contained in(plugging) CLIP(to cut the edges off/trim).

17. Imminent selection of trains to Retford (2,5)
IN STORE : Hidden in(selection of) “trains to Retford”.
18. Updates can’t corrupt word-processing technique (3,3,5)
CUT AND PASTE : Anagram of(… corrupt) UPDATES CAN’T.
Defn: … as part of computer software.
22. Composition of other ranks taken on by Nelson without introduction (8)
ORATORIO : OR(abbrev. for”other ranks” in the military) contained in(taken on by) “Horatio”(Nelson, once admiral of the British Navy) minus(without) its 1st letter(introduction).
Defn: A musical … for orchestra, choir and vocal soloists that narrates a story.
23. Compete with sides from Nigeria outside northern capital (6)
VIENNA : VIE(to compete/contest with) + 1st and last letters of(sides from) “Nigeria” containing(outside) N(abbrev. for “northern”).
Defn: … of Austria.
24. Pop round, given small desire for food (8)
APPETITE : Reversal of(… round) PA(like “pop”, a familiar term for one’s father) plus(given) PETITE(small and trim, refering to a woman).
25. Venerate American patriot Paul (6)
REVERE : Double defn: 1st: …/regard with reverence/respect); and 2nd: … during the American Revolutionary War.
Down
1. Terrible wind interrupts middle of meal. Curry perhaps? (6)
EDWINA : Anagram of(Terrible) WIND contained in(interrupts) middle 2 letters of(middle of) “meal”.
Defn: A homophonic reference/perhaps? to Edwina Currie, former UK Junior Health Minister and controversial figure.
2. The morning after ejecting good Hislop, say, from a royal house (10)
HANOVERIAN : “hangover”(the unpleasant physical and psychological effects after a night of heavy drinking/“the morning after”) minus(ejecting) “g”(abbrev. for “good”) + IAN(Hislop, British TV personality, an example/say of someone with that given name).
Defn: Relating to the German royal house whose members formerly ruled Great Britain at various times.
3. Overturned legal action about new fool’s complaint in hearing (8)
TINNITUS : Reversal of(Overturned, in a down clue) SUIT(a legal action brought to court) containing(about) [ N(abbrev. for “new”) + NIT(a nitwit/fool) ].
Defn: …, namely, a ringing or other noise heard in one’s ear(s) that is not from the outside environment.
4. Discrepancy is holding up male striker? (8)
MISMATCH : [ Reversal of(… up, in a down clue) IS placed below((… holding, in a down clue) M(abbrev. for “male”) ] + MATCH(a short chemical-tipped stick that produces a flame when struck, therefore what you might call a striker).
5. OK, USA angry with raw Japanese film-maker (8)
KUROSAWA : Anagram of(… angry …) [OK, USA plus(with) RAW].
Answer: Akira …, prolific and influential Japanese film-maker.
7. Crash into party (4)
BASH : Double defn: 2nd: A lively and large celebratory event/….
8. Regularly observed Neptunian volcano (4)
ETNA : 2nd, 4th and 6th letters of(Regularly observed) “Neptunian”.
Defn: … in Sicily.
12. English pirate confused with tax exile (10)
EXPATRIATE : E(abbrev. for “English”) + anagram of(… confused) [PIRATE plus(with) TAX ].
Answer: Historically used to refer to an exile, one who is banished from their country. (In modern usage it refers to one moving voluntarily and temporarily to another country for various reasons.)
13. Rubbish collection of points almost nobody accepts (8)
NONSENSE : S,E,N,S(collection of abbrevs. for the compass points, south, east and north except west/almost) contained in(…accepts) NONE(nobody/no one)
OR
S,E,N,S,E(collection of abrevs. for the compass points, south, east and north) placed below(… accepts, in a down clue – though “accepts” is usually a containment indicator) “none”(nobody/no one) minus its last letter(almost …).
14. Create classes in biology on vacuous theme (8)
GENERATE : GENERA(classes/divisions of closely related species in biology) placed above(on, in a down clue) 1st and last letters deleted from(vacuous) “theme”.
16. Track automatically not changed after secure start to Olympics (4,2,2)
LOCK ON TO : Anagram of(… changed) NOT placed below(after, in a down clue) [ LOCK(to secure/make fast) + 1st letter of(start to) “Olympics” ].
19. Shrub split in part (6)
PRIVET : RIVE(to split/tear apart) contained in(in) PT(abbrev. for “part”).

20. Increasingly bright star Victor wears angora, off and on (4)
NOVA : V(letter represented by “Victor” in the phonetic alphabet) contained in(wears) 2nd, 4th and 6th letters of(…, off and on) “angora”.
21. Finally evading snake, breathe with effort (4)
GASP : Last letter of(Finally) “evading” + ASP(a venomous snake).
I thought that 13d was a bit of a lazy clue – I’m sure there are other/better ways to clue SENS?
New for me: journalist Ian Hislop (for 2d).
Is a SARNIE always made with bread rolls as pictured above, or can it also be a sarnie if it is made with slices of bread? I have heard of sarnies but don’t really know what they are, ditto butties and baps (and too lazy to google them but I might one day!)
Thanks scchua. Your second parsing of NONSENSE makes more sense, though as you said, accepting is usually an insertion indicator. Also, my gut feeling is that Harpo didn’t check the spelling of Currie for EDWINA – “perhaps “ for a soundalike indicator doesn’t work for me.
Michelle @1 SARNIE is interchangeable with sandwich. I would have pictured slices of bread being used, but you can use other bread products to make sandwiches. These days the filling + bread product format is often used in cafés when it’s not slices eg. Ham and cheese roll or coronation chicken baguette. A bap is a specific type of roll that people around the UK call different things and are tricked into arguing about on social media.
I got off to a slow start here. I sometimes do the main cryptic in less time, although we should expect some crossover.
I registered the Currie problem but only as a passing thought. I didn’t know the book, but was confident with MIKADO. There was maybe more general knowledge required than usual.
Thanks Harpo and scchua
Michelle – Further to Martin @3 “buttie” is just another name for a sarnie, although many years ago somebody told me that a sandwich is made with two slices of bread, but a buttie is made with a slice of bread folded in half.
Another Sunday cryptic from Harpo; luckily I’ve been able to complete it this time, having had to construct some jorums, including TINNITUS and EDWINA (as it turns out, plainly a wrong clue). VARIABLE is a non-cryptic clue. Overall though, with the exception of vocabulary, I felt this was quipticky enough. My favourites were MIKADO and COWSLIP. Thanks Harpo and scchua!