Thank you to Mudd. Definitions are underlined in the clues.
Across
1. Cross-channel boat, transport reversed into by judge (3,5)
CAR FERRY : CARRY(to transport/convey) containing(… into by) reversal of(reversed) REF(short for “referee”, judge in a competitive match).
5. Grant set off in front (6)
BESTOW : Anagram of(… off) SET contained in(in) BOW(the front end of a ship).
10. Capital of Lesotho is initially seen on holiday (7)
TRIPOLI : 1st letters, respectively, of(… initially) “of Lesotho is” placed after(seen on) TRIP(a holiday/a going-away for pleasure).
Defn: … of Libya.
11. Pub drink, did you say? Jump the queue! (5,2)
BARGE IN : Homophone of(…, did you say) “bar”(pub/establishment selling drinks) “gin”(an alcoholic drink).
12. Leader eliminated on Mediterranean island in aloof manner (5)
ICILY : 1st letter deleted from(Leader eliminated on) “Sicily”(island in the Mediterranean Sea).
13. Numbered group goes with them, I gathered (9)
EIGHTSOME : Anagram of(… gathered) [GOES plus(with) THEM, I].
14. Coronation artefact, rock cake? (5,2,5)
STONE OF SCONE : STONE(a rock/piece of hard solid material) [OF SCONE](relating to “scone”, a small unsweetened or lightly sweetened cake).
I thought “rock cake” should, grammatically, lead to “Scone of stone”.
Defn: …, viz. an oblong block of sandstone formerly used in the coronations of Scottish, then English, then British monarchs.
… that’s it under the seat.
18. Honorary title easy, as my lord is decorated (4,8)
LADY MAYORESS : Anagram of(… is decorated) EASY, AS MY LORD.
21. Bud’s opening in high plant (9)
EUPHORBIA : 1st letter of(…’s opening) “Bud’” contained in(in) EUPHORIA(a feeling of intense excitement and happiness/a high).
One of many species:

23. Portrait that is on the outside of publication (5)
IMAGE : IE(abbrev. for “id est”/that is) containing(on the outside of) MAG(short for “magazine”, a publication).
24. River in South America, alternatively in old Congo, drained (7)
ORINOCO : OR(a conjunction linking options/alternatively) + IN + O(abbrev. for “old”) + all inner letters deleted from(…, drained) “Congo”.
25. Cinema travelled to, start eating, rustling initially muffled (5-2)
DRIVE-IN : [DIVE IN](to start eating food) containing(… muffled/wrapped) 1st letter of(… initially) “rustling”.

26. Figure with sore back, painful to touch (6)
TENDER : TEN(a number/figure) plus(with) reversal of(… back) RED(describing part of one’s body that is inflamed/painful/sore).
27. At once sister is working after beginning on itinerary (2,6)
IN UNISON : [ NUN(a member of a religious order of women/sister) + IS + ON(working/operating) ] placed after(after) 1st letter of(beginning on) “itinerary”.
Defn: …/simultaneously, as in “There is no way he could do all those things at once”.
Down
1. Blooming cluster — Tom, his wife and kittens? (6)
CATKIN : CAT(the male of which animal is a “tom”) + KIN(one’s family, in this case, the wife and kittens of a tom).

2. Fruit farmin’ (6)
RAISIN : “raising”(breeding or growing of animals and plants/farming) minus its last letter as in “farmin’”.
3. Finally, people stop supporting comfortable living arrangement (9)
ECOSYSTEM : Last letter of(Finally) “people” + [ STEM(to stop/to restrain) placed below(supporting, in a down clue) COSY(comfortable/at ease) ].
4. A New Year sober, I suspect? Look surprised! (5,2,7)
RAISE AN EYEBROW : Anagram of(… suspect) A NEW YEAR SOBER, I.
6. Heron nesting, in the end, in different tree (5)
EGRET : Last letter of(…, in the end) “nesting” contained in(in) anagram of(different) TREE.
7. In direct conflict, as one progresses along line of piggies? (3-2-3)
TOE-TO-TOE : What you might call the progression from one toe/piggy to the next piggy and so on.
Defn: …, figuratively, from the literal description of two people standing directly and closely in front of one another, as if ready to fight or argue.
8. Rover I’m not sure will go twice after stick (8)
WANDERER : ER,ER(twice the expression for “I’m not sure/I hesitate”) placed below(will go … after, in a down clue) WAND(a rod/a long thin stick).
9. Will you excuse me, huh? (1,3,4,6)
I BEG YOUR PARDON : Double defn: 1st: Like “…”, an expression of polite apology; and 2nd: Like “…!”, an expression of anger.
15. Cary Grant movie that’s funny is on, American film screened (9)
SUSPICION : Anagram of(funny) IS ON containing(… screened) [ US(abbrev. for things American) + PIC(short for “picture”, in this case, a cinema film/movie) ].
16. Fish after fish served up — be a kipper elsewhere? (5,3)
SLEEP OUT : POUT(a seafish of the cod family) placed below(after, in a down clue) reversal of(… served up, in a down clue) EELS(snake-like fish).
Defn: To kip/sleep away from home.
17. Fuss over moot point, choice of candidate (8)
ADOPTION : ADO(a fuss/commotion) placed above(over, in a down clue) + anagram of(moot/questionable) POINT.
Defn: Act of choosing someone as a candidate for office/nomination.
19. Requirement for mash taster after mashing! (6)
TATERS : Anagram of(… after mashing) TASTER.
Answer: …/informal term for potatoes, an essential ingredient, after boiling and mashing, of mashed potatoes/mash.
20. Marker and name on flag (6)
PENNON : PEN(device that can be used to mark/a marker) plus(and) N(abbrev. for “name”) + ON.

22. Weight and number for gas (5)
OZONE : OZ(abbrev. for “ounce”, a unit measure of weight) plus(and) ONE(a number).
Fun puzzle. Nho of pout as a fish.
Enjoyed the illustrated blog.
Thanks Mudd and scchua.
Thanks both, straightforward fun. Liked lady mayoress, raise an eyebrow and others.