Thank you to Bobcat. Definitions are underlined in the clues.

Across
1. Cat a waterbird mostly observed going round tree (8)
BERGAMOT : Reversal of(… observed going round) [ TOM(a male domestic cat) + A + “grebe”(a small to medium-sized waterbird) minus its last letter(mostly) ].
Answer: A citrus fruit tree.

9. ICE about to intercept worker arriving at American ranch (8)
HACIENDA : Anagram of(… about) ICE contained in(to intercept) HAND(a worker doing physical or manual labour) plus(arriving at) A(abbrev. for “American”).
Defn: … in Spain and former Spanish colonies.
10. Turned out dire heads in middle of March (4)
DEMO : Inner letters deleted from(Turned out/emptied a container of its contents) “dire” + 1st letters, respectively, of(heads in) “middle of”.
Answer: Short for “demonstration”, a protest march.
11. Suffering duress, United must supply reserves (12)
UNDERSTUDIES : Anagram of(Suffering) DURESS, UNITED.
Defn: …/stand-ins.
13. Description of movement up in certain circles (6)
PINCER : Hidden in(… circles) “up in certain”.
Answer: A military tactic where the enemy is attacked on both flanks at the same time.
14. Defector shot commanding officer about to be discharged (8)
TURNCOAT : TURN(the chance, in rotation amongst other people, to do something/a shot) + CO(abbrev. for “Commanding Officer”) + “about” minus its inner letters(to be discharged/unloaded).
15. Catty sort of secretary doing duty for Tesla’s outside in skintight clothing (7)
LEOPARD : PA(abbrev. for “personal assistant”, a secretary) replacing(doing duty for) 1st and last letters of(…’s outside) “Tesla” in “leotard”(skintight one-piece clothing).
16. Metropolitan spread straddles border (7)
PRIMATE : PÂTÉ(a savoury spread of chopped or pureed meat) containing(straddles) RIM(a border/edge).
Defn: …/a high-ranking bishop in Christianity.
20. A case of the Home Counties having a calming effect? (8)
SEDATIVE : [ SE(abbrev. for “South East”, representing the counties in southeast England around London, or the Home Counties) DATIVE(an example of a case, a grammatical form of a noun or pronoun) ](what you might call a case of the Home Counties).
22. Golf club’s right wing becoming Liberal? That’s nonsense (6)
DRIVEL : Last letter of(…’s right wing, in an across clue) “driver”(a golf club used to hit the ball off the tee) replaced by(becoming) L(abbrev. for a member of the Liberal Party).
23. Fashion designs never wear (7,5)
EVENING DRESS : Anagram of(Fashion) DESIGNS NEVER.
Defn: …/an item of clothing.
25. What former manager says she did for this country (4)
IRAN : “I RAN”(what a former manager of, say, an organisation says she did).
26. Impressive European politician has curtailed involuntary movement (8)
EMPHATIC : E(abbrev. for “European”) + MP(abbrev. for a member of Parliament, a politician) + “has” minus its last letter(curtailed) + TIC(an involuntary muscle movement/spasm).
Defn: …/having a forceful effect.
27. Order Charlie, lying about being confronted by head, to omit nothing (8)
TIDINESS : [ Reversal of(…, lying about) “idiot”(a fool/a Charlie) placed before(being confronted by) NESS(a narrow strip of land projecting into a body of water/a head) ] minus(to omit) “o”(letter representing 0/nothing).
Down
2. Tax catches King Edward in drill (8)
EXERCISE : EXCISE(tax/duty collected on specific goods) containing(catches) ER(abbrev. for “Edward Rex”/King Edward).
3. Someone to take high level command? Count Agrippa out (5,7)
GROUP CAPTAIN : Anagram of(… out) COUNT AGRIPPA.
Answer: A senior officer in the Air Force, with the latter having literally high level/flying components.
4. Criminal heading north on famous horse (8)
MURDERER : Reversal of(heading north, in a down clue) [ RE(with reference to/on) + RED RUM(a famous champion horse who won 3 Grand Nationals) ].
5. Dehydrated, tense and ill-tempered, beginning to drop (7)
THIRSTY : T(abbrev. for “tense” in grammar) plus(and) “shirty”(irritable/ill-tempered) with its 1st letter(beginning) moving to a lower position(to drop, in a down clue).
6. Bird coming out of racecourse paraded topless before old queen (6)
SCOTER : “Ascot”(a racecourse in England) minus its 1st letter(paraded topless) plus(before) ER(abbrev. for “Elizabeth Regina”, either Queen Elizabeth I or II, a former queen).
7. One who’s against first couple of ancient relations turning up (4)
ANTI : 1st 2 letters of(first couple of) “ancient” + reversal of(… turning up, in a down clue) IT(euphemism for sexual intercourse/relations).
8. This tape could be used, with student’s input, to make telecasts (8)
CASSETTE : Reverse clue: Anagram of(… could be used) CASSETTE containing(with …’s input) L(plate displayed by a learner/student driver) =(to make) “telecasts”.
Remember these?

12. Poor Domenico Scarlatti gutted — mass regularly put out of service (12)
DECOMMISSION : Anagram of(Poor) [DOMENICO + “Scarlatti” minus its inner letters(gutted) + 1st and 3rd letters of(… regularly) “mass” ].
15. £1 notes, not one new, in circulation, I hear (8)
LISTENER : LI(the British pound from the abbrev. for “libra” + Roman numeral for 1/£1) + [“tenners”(informal term for the British ten-pound/£10 notes) minus(not) one “n”(abbrev. for “new”) with its last letter moved to the beginning(in circulation) ].
17. Pairs finishing chore made stratagem, escaped, and got back (8)
REDEEMED : Last 2 letters, respectively, of(Pairs finishing) “chore made stratagem, escaped”.
Defn: …/recovered, say, one’s possession by paying off a debt.
18. Spring lock checks unlawful entry (8)
TRESPASS : SPA(a spring/source of water containing minerals) contained in(checks) TRESS(a lock of hair)
19. Mounted police to charge public: leader’s lacking judgement (7)
VERDICT : Reversal of(Mounted, in a down clue) CID(abbrev. for the Criminal Investigation Department, a branch of the police force) contained in(to charge) “overt”(public/not concealed from general view or knowledge) minus its 1st letter(leader’s lacking).
Defn: … in, say, a court of law.
21. Logs initially barred from coal fire (6)
IGNITE : 1st letter of(… initially) “Logs” deleted from(barred from) “lignite”(a type of coal).
Defn: …/to cause to burn.
24. Show old mate where to go at night? (4)
EXPO : EX(former/old mate/partner) + PO(informal word for “chamberpot”, a vessel to urinate into/go at night in the bedroom).
Answer: An event where goods are displayed/shown.
Thanks for the blog , very good set of clever clues with a lot of variety . I spotted the LEOPARD but not the tigress .
Unusual grid again with eight entries having double unchecked , but half overall checked so fair enough .
Minor error in blog for THIRSTY. The S isn’t removed but is dropped to a lower position.
Really enjoyed this challenge and very good blog too. Thank you Bobcat and scchua
Good puzzle. The top half went in quickly – bottom half took longer.
Favourties were: EMPHATIC, PINCER, HACIENDA
Thanks Bobcat and scchua
Thanks Hovis for pointing it out. Blog corrected.
Lots of variety today, as already noted by Roz. I thought it had some great clues, but so many words also made the experience draining.
I liked HACIENDA for its nice, but sad, surface. I loved the wonderful anagrams in 3, 23 and 16 and also ticked DRIVEL
Thanks scchua for the colourful blog and, more importantly, for clearing up several parsing mysteries. Thanks Bobcat for the puzzle.
Great puzzle. I had the classic experience of finding it opaque at first, going away to do other things, and coming back to find it all went along nicely.
Second what others have said… definitely needed the blog to explain some, eg LISTENER, I would not have thought of notes as TENNERS either before or after solving… had to look up the duck to confirm, also… some crafty stuff, which I enjoyed a lot HACIENDA, and VERDICT probably get my vote…
Thanks Bobcat n scchua
Late today. I needed the blog for PO=chamberpot, a new one for me. The tigress helped me find a few of the solutions on the left-hand side. Bobcat always required some lateral thinking, so I do appreciate the cats.
HACIENDA has a sadly topical surface. (Make no mistake: many Americans are…not happy with what’s going on, to put it mildly.) Also worth noting that among the former Spanish colonies where there were haciendas is Texas, so it could indeed be an American ranch.