Thank you to Filbert. Definitions are underlined in the clues.
Across
1. Ship’s officers make fast haul to port (8)
WARDROOM : Reversal of(… to port/to the left, in nautical language) [ MOOR(to make fast/to tie a ship to, say, a dock) + DRAW(to haul/to pull along) ].
Defn: …, so called from the mess/compartment in a warship that they have the right to occupy.
6. Moth damaged by flame keeps back from candle (6)
BURNET : BURNT(damaged by flame/fire) containing (keeps) last letter of(back from) “candle”.

9. Rather like that ruined fort (4,2)
SORT OF : SO(like that/in such a way) + anagram of(ruined) FORT.
10. Line of railway on winding ascent (8)
ANCESTRY : RY(abbrev. for “railway”) placed after(on) anagram of(winding) ASCENT.
Defn: … of descent/a connected series of people following one another in time across several generations.
11. Writer casting doubt on William the Conqueror’s armour? (6,6)
NORMAN MAILER : ER(an expression of a feeling of doubt/uncertainty) placed after(on) [ NORMAN(a member of a people, of which William the Conqueror was an example) + MAIL(armour made of joined metal rings or plates) ].
Defn: American …
.
13. Clergyman‘s runs at some point stop (8)
REVEREND : R(abbrev. for “runs” in cricket scores) + EVER(at any/some point) + END(stop/cease).
15. Horrible king who succeeded Anne by scheming (6)
GRISLY : GRI(abbrev. for George Rex I, British King George the 1st, with the Roman numeral substitution, who succeeded Queen Anne) plus(by) SLY(scheming/cunning).
16. Cheap writer pockets 10p; that’s life, you see (6)
BIOPIC : BIC(or Bic Biro, a cheap/inexpensive disposable ball-point pen/writer) containing(pockets) [ I(Roman numeral for 1) + O(letter representing 0) + P(abbrev. for “penny”, the British unit of currency) ].
Defn: A film that you might see depicting the life of a particular person.
18. Obvious one’s last in line for extra pay (8)
OVERTIME : OVERT(obvious/done or shown openly) + I’M(“I am”/one is/one’s, speaking self-referentially) + last letter of(last in) “line”.
20. First Lady unhappily married to Head of State? (5,7)
DIANA SPENCER : Cryptic defn: The first lady/first wife of King Charles III, current UK Head of State, who was in an unhappy marriage.
23. Last defeat playing Real, two nil (8)
WATERLOO : Anagram of(playing) REAL, TWO + O(letter representing 0/nil).
Defn: Final/…, from the battle which ended Napoleon I’s regime.
24. Get back home after concerning case of gonorrhoea (6)
REGAIN : IN(home/not out) placed after(after) [ RE(with reference to/concerning) + 1st and last letters of(case of) “gonorrhoea” ].
25. Spirit lives on after (6)
PASTIS : IS(lives/exists) placed after(on) PAST(after, when specifying time, as in “ten minutes past ten”).
Defn: …/liqueur flavoured with aniseed.
26. Wandering sheep interrupts general’s planning (8)
STRATEGY : STRAY(wandering/roaming) containing(… interrupts) TEG(a two-year old sheep).
Defn: A plan of action a military general might devise.
Down
2. Stretching account with car purchase, one’s sorry (10)
APOLOGETIC : AC(abbrev. for “account”) containing(Stretching … with) [ POLO(a model of car manufactured by Volkswagen) + GET(to purchase/buy) + I(Roman numeral for “one”) ].
3. Useful number mother used to get hold of trade union (5)
DATUM : DAM(mother/female parent of an animal) containing(… used to get hold of) TU(abbrev. for “trade union”).
Defn: …, in this case a numerical data point which can be used as a benchmark.
4. Injury from fight with swords (7)
OFFENCE : OF(from/as a result of, as in “he died of cancer”) + FENCE(to fight with swords).
Defn: …/something that hurts one’s feeling, an insult,, say.
5. A potentate so mad might become commonplace in America (4-3-8)
MEAT-AND-POTATOES : Anagram of(… might become) A POTENTATE SO MAD.
Defn: …/Americanism for describing the simple and essential parts of something.
6. Things piled up in in-tray have nowhere to go, according to Spooner (7)
BACKLOG : Spoonerism of(…, according to Spooner) “lack bog”(without a toilet, thus having nowhere to go/urinate or defecate).
7. Check school during break (9)
RESTRAINT : TRAIN(to school/to teach) contained in(during) REST(a break/a pause from working).
8. Land before crossing island (4)
EIRE : ERE(before a specific time) containing(crossing) I(abbrev. for “island”).
Defn: …/nation.
12. Son clumsily walking in sleep (10)
SLUMBERING : S(abbrev. for “son”) + LUMBERING(clumsily/slowly and awkwardly walking).
14. I peed next to doctor for convenience (9)
EXPEDIENT : Anagram of(… to doctor) I PEED NEXT.
Answer: …, as a noun.
17. 50% chance most of heaven’s white (7)
CHABLIS : 1st 3 letters out of 6 of(50%) “chance” + “bliss”(heaven/perfect happiness) minus its last letter(most of …).
Defn: … wine.
19. Big head – Caesarean, possibly (7)
EMPEROR : I’m guessing it’s a double defn: 1st: What you might call a …/an arrogant person, or, a big/sovereign head/ruler; and 2nd: Possibly/any of the sovereign rulers of the Roman Empire who used the title, Caesar.
And is the surface an allusion to a C-section?
21. Imminent threat’s close hours before alarm sounds (5)
NIGHT : NIGH(imminent/happening soon) + last letter of(…’s close) “threat”.
Defn: The …, that is, the period before your morning alarm clock sounds.
22. One might look after the kids twice a year (4)
PAPA : 2 x(twice) PA(abbrev. for “per year”/a year).
Excellent puzzle. Couldn’t see the reversal in 1a (must be blind) so thanks Mr Blog and I wondered about 20a but CD sounds fine. Perfect Sunday morning tussle. Likes for APOLOGETIC, GRISLY, and NIGHT among others. Thanks schuaa (lovely pic) and FILBERT.
My top faves: WARDROOM, GRISLY and BACKLOG.
OVERTIME
Is the ‘for’ required as a part of the def?
EMPEROR
Nice surface (agree with the blogger’s comment).
One question: Do the words ‘Caesarean, possibly’ not lead to
‘of EMPEROR’ rather than to ‘EMPEROR’?
Thanks Filbert and scchua.
Faves: WARDROOM, ANCESTRY, GRISLY, WATERLOO, STRATEGY, APOLOGETIC, RESTRAINT, SLUMBERING and CHABLIS.
I’m afraid I didn’t get either DIANA SPENCER or EMPEROR but that is often the way with cryptic definitions. And I don’t see how ‘doctor’ can work as an anagrind after the fodder in EXPEDIENT.
Thanks Filbert and scchua
KVa@2 – Not sure what you mean about 19d EMPEROR.
FrankieG: I think KVa is noting that ‘Caesarean, possibly’ is an adjectival phrase whereas EMPEROR is a noun so the def is describing rather than defining the solution? Maybe the clue is intended to be more of an extended cryptic definition?
I had ‘Caesarian possibly’ meaning either Augustus or Julius among the roll call of Roman emperors without worrying about any grammatical niceties and I had the anagrind in 14d as ‘to doctor’ which I think works fine retrospectively.
PostMark@5
EMPEROR
Yes. That’s what I meant to ask (Kind attention: FrankieG@4).
I’m still struggling with 19d. Aside from the Caesarian part, I don’t see EMPEROR as meaning someone who is big headed. Possibly it works in the sense of “big chief”, or even as “big” and “head” separately?
But nothing to spoil an excellent Sunday morning puzzle.
Thanks to Filbert and scchua
I felt that the head of an empire would be a ‘big head’ and could possibly be Caesarean. The clue for Diana Spencer seems very odd to me.
Thanks Filbert for a fine set of clues. My favourites were WARDROOM, ANCESTRY, BIOPIC, REGAIN, PASTIS, and SLUMBERING. I correctly entered DIANA SPENCER and EMPEROR with a shrug and used a word finder for APOLOGETIC and GRISLY, neither of which I could parse. Thanks scchua for the blog.
Thanks both. Very late to start and finish, so with some hesitation cutting to the chase to say this was generally the high standard the setter always achieves, but EMPEROR and DIANA SPENCER don’t quite work for me, the first for reasons already discussed here, the second for the small matter that the cryptic definition potentially implies the second marriage is also unhappy, yet it didn’t spot me justifying the answer once the crossers suggested the answer.
Thanks Filbert and scchua. I too am a bit unsatisfied with 19 as a DD. Assuming the definition is “Big head”, perhaps the “dash” in the clue is significant. If we take this as an EM dash, we can couple it with the Latinish (“Caesarean”) phrase PER OR, which could, cum grano salis, be taken to mean “by way of alternative” = “possibly”. Just a shot …
Caesarean can be a noun: “1. 1528– An adherent of Caesar, of the Emperor (against the Pope), or of an imperial system.
2. 1923– (Also with lower-case initial.) A Caesarean section. colloquial.
… 1985 The AMA reports that a Caesarean commands an additional £250 over a vaginal delivery. Washington Post 13 January” — “£” not “$”?