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) minus its 1st letter(beginning to drop).
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