Thanks to Gurney for an enjoyable puzzle. Definitions are underlined in the clues.
Across
9 Eyeing from here, proves boat is not manoeuvring? (11,4)
OBSERVATION POST : Anagram of(… manoeuvring) PROVES BOAT IS NOT.
10 Expression of contempt about colour (5)
OCHRE : OCH!(in Scottish and Irish, an expression of disapproval/contempt) + RE(about/with reference to).
11 Reportedly quiet before lunch, say, patchy (9)
PIECEMEAL : Homophone of(Reportedly) “peace”(quiet) plus(before) MEAL(an example of which/say, is lunch).
12 He takes meat in refurbished Rio cavern (9)
CARNIVORE : Anagram of(refurbished) RIO CAVERN.
14 After tips from teetotaller rejected fish (5)
TROUT : 1st and last letters of(tips from) “teetotaller” plus(After …) OUT(rejected/dismissed).
16 On return, emphasised farmers’ body is in sharing arrangement – sweet (a bit perhaps) (15)
DESSERTSPOONFUL : Reversal of(On return) STRESSED(emphasised/gave particular importance to a point, statement, idea, or syllable in speech or writing) + [NFU(abbrev. for the National Farmers’ Union/body/organisation) contained in(is in) POOL(an agreement for contributors to share usage of their resources, say, cars, finances, etc).
Defn: A small amount/a bit of sweet/dessert measured in a spoon for that purpose, smaller than a tablespoon and larger than a teaspoon.
19 Perfume despatched, by sound of it (5)
SCENT : Homophone of(…, by sound of it) “sent”(despatched/forwarded).
21 A place for bowling impresses writer – time for correspondence (9)
AGREEMENT : A + GREEN(a place/the playing area, specifically a closely trimmed lawn, for the game of bowls) containing(impresses) ME(the writer of this clue, using the self-referential pronoun) + T(abbrev. for “time”).
23 Dined alone cut off in plant (9)
DANDELION : Anagram of(… off) [DINED + “alone” minus its last letter(cut off)].
25 Coming back, make fun of our group? That’s annoying! (5)
SUGAR : Reversal of(Coming back) [RAG(to make fun of/to tease) + US(our group, in contrast to “them”).
Defn: A euphemism for “shit”, as an expression of annoyance, as in “Oh sugar! I just shat (or sugared??) in my pants!”
26 Note usual triers messing about – dangerous game (7,8)
RUSSIAN ROULETTE : Anagram of(… messing about) NOTE USUAL TRIERS.

Down
1 Democrats after review accepting old people, important, may move in these (10)
MOTORCADES : Anagram of(… after review) DEMOCRATS containing(accepting) O(abbrev. for “old”).
Defn: VIPs/important people may move/parade (or be paraded) in these.

2 To some extent likes the rackety girl (6)
ESTHER : Hidden in(To some extent) “likes the rackety“.
3 Note about dance at home – no palms beyond this? (4,4)
TREE LINE : TE(ti/the syllable representing the seventh note in the sol-fa musical scale) containing(about) [REEL(a lively Scottish or Irish folk dance) + IN(at home/not outdoors).
Defn: The altitude on a mountain, or the northern or southern latitudes beyond which trees, whether palms or other trees, cannot grow because of the extreme environment.
4 Not exactly politician’s military life (4)
CAMP : CA(abbrev. for “circa”/about/not exactly when refering to specific historical points in time) + MP(abbrev. for “Member of Parliament”/a politician).
5 Row about Maine’s abbreviation – look up and watch maybe (10)
TIMEKEEPER : TIER(a row/an arranged line of similar objects) containing(about) [ME(abbrev. for the US state of Maine) + reversal of(… up, in a down clue) PEEK(to look quickly or slyly)].
6 Popular group fly perhaps (6)
INSECT : IN(popular/in fashion) + SECT(a sub group of people within, or an offshoot of, a bigger group in a religious, political or philosophical belief system.
Defn: …/is an example of, this.
7 No mixer – learner, individual needing smooth progress on way up (4,4)
LONE WOLF : L(letter associated with, and displayed by, a learner driver) + ONE(an individual/a singular person) plus(needing) reversal of(… up, in a down clue) FLOW(smooth progress on way/a continuous and steady streaming along).
8 From metallics and others (2,2)
ET AL : Hidden in(From) “metallics“.
13 Overnight case? No (10)
OUTPATIENT : Cryptic defn: Not luggage, but a sick person/a medical case who visits a hospital but is not warded/does not stay overnight.
15 Frankly discuss story finally changed with support of country (4,6)
TALK TURKEY : “tale”(a story) with its last letter changed(finally changed) placed above(with support of, in a down clue) TURKEY(the Mediterranean country).
17 Back in Paris is becoming rigorous in the extreme (8)
STERNEST : STERN(the back end of, say, a ship) + EST(“is” in French, the language spoken in Paris).
18 Exaggerated merit of completed series’ introduction – out of date (8)
OVERSOLD : OVER(completed/done with) + 1st letter of(…’ introduction) “series” + OLD(out of date/obsolete).
20 Unproved premise in article so idealistic, silly first of all (6)
THESIS : THE(an article in grammar) + 1st letters, respectively, of(… first of all) “so idealistic, silly“.
22 Describing position of summit, beginning to end (6)
EIGHTH : HEIGHT(a summit/the highest point of a mountain, say) with its 1st letter(beginning) moved to the last(to end).
Defn: … of a specific item in the order of a series.
23 Read about challenge (4)
DARE : Anagram of(… about) READ.
24 Type of light knee joint, nothing odd in that (4)
NEON : “knee joint” minus its 1st, 3rd, 5th, 7th and 9th letters(nothing odd in that).

Thanks scchua. I found this quite a lot harder than the usual run of Gurney puzzles, but immensely enjoyable, so all the better for that. Thanks Gurney.
Thanks to Gurney and scchua. I parsed but did not understand SUGAR but was defeated by AGREEMENT and OUTPATIENT. Still, an enjoyable puzzle.
Thanks Gurney & scchua.
I solved all except “eighth”.
Two observations: In 23 across, “off” need not be repeated; and in 25 across “sugar!” is a euphemism for “bugger!”.
Thanks Gurney and scchua
Started this one over brekkie and couldn’t get it finished and needed a couple of other shorter sessions to get it done. Found myself writing in a couple of wrong answers initially – OBSERVATION SPOT at 9a and THEORY at 20d that didn’t help the cause.
Thought that SUGAR was very clever when the penny dropped … it was the second last one in, just after LONE WOLF (which was also clever) and followed by OUTPATIENT (which was the best of them all).
Many thanks, scchua, for the excellent blog and many thanks also to all who commented.
I did not get 22d; but I did get 13d: both brilliant