Thank you Hectence for an enjoyable Quiptic. (Aptly relaxing for Labour Day.) Definitions are underlined in the clues.
Across
1 Don’t allow backing for Trump’s extremes to overshadow right’s regular following (7)
PREVENT : Reversal of(backing for) the 1st and last letters of(…’s extremes) “Trump” containing(to overshadow) [ R(abbrev. for “right”) plus(… following) EVEN(regular;unvarying) ].
5 Most unhappy when daughters break new seats (7)
SADDEST : D(abbrev. for “daughter”) twice(…s) contained in(break) anagram of(new) SEATS.
9 Clown has a drink before bed (5)
COCOA : COCO(the Clown, a well-known circus performer in the UK around the 50s) plus(has) A.
10 Overhear report of first lady’s decline (9)
EAVESDROP : Homophone of(report of) [ “Eve”(the first lady in the Bible)‘s “drop”(a decline;a fall) ].
11 Same silly nonsense without any purpose (9)
AIMLESSLY : Anagram of(… nonsense) SAME SILLY.
12 Vocal about going ahead to see reef creatures (5)
CORAL : ORAL(vocal, in contrast to “written”) placed after(… going ahead) C(abbrev. for “circa”;about, when used in conjunction with dates).
13 Demand English vote on legislation (5)
EXACT : E(abbrev. for “English”) + X(letter indicating a vote for, say, an election candidate) plus(on) ACT(legislation by, say, Parliament).
15 Does up old veteran’s junk (9)
RENOVATES : Anagram of(… junk) [ O(abbrev. for “old”) + VETERAN ].
18 Cast doubt on record and run revision (9)
DISCREDIT : DISC(a phonograph record) plus(and) R(abbrev. for “run” in cricket scores) + EDIT(as a noun, a revision of piece of text).
19 Get to share a cheese sample (5)
REACH : Hidden in(… sample) “share a cheese “.
21 Bruised head’s sore with long fight (5)
BRAWL : The 1st letter of(… head) “Bruised” + RAW(sore;red and painful) + L(abbrev. for “long”, as in LT for the unit of weight, the long ton).
23 Wearisome fellow at first of Ireland’s games nursing a half of Guinness (9)
FATIGUING : F(abbrev. for “fellow”) + AT + [ the 1st letters, respectively, of(first of) “Ireland’s games” co0(nursing) the 1st four letters, out of eight, of(a half of) “Guinness” ].
25 New Yorker, say, has love for black coffee … (9)
AMERICANO : AMERICAN(an example of which could be;say, a New Yorker) plus(has) O(letter signifying 0;love in tennis scores).
26 … taking a fancy to similar! (5)
ALIKE : A + LIKE(a fancy, as in “one’s likes and dislikes”].
27 Man on board is a breath of fresh air (7)
DRAUGHT : Double defn: 1st: … or draughtsman, one of the pieces in the board game of draughts.

28 Cost is 10p, we hear (7)
EXPENSE : Homophone of(…, we hear) [ “X”(Roman numeral for 10) “pence”(the currency units, abbreviated as “p”) ].
Down
1 Time to support group deal (7)
PACKAGE : AGE(a long period of time) placed below(to support, in a down clue) PACK(a group, say, of people or animals).
Defn: Something offered, say, a bargain;deal as a combination of various things, eg. a holiday package.
2 Surround with trained men to guard company on way through mountains (9)
ENCOMPASS : [ Anagram of(trained) MEN containing(to guard) CO(abbrev. for “company”;a commercial firm) ] placed above(on, in a down clue) PASS(a way;a route through;between mountains).
3 Times has note to delete (5)
ERASE : ERAS(multiple long periods of time) plus(has) E(note representing the third tone in the music scale of C major).
4 Valued guaranteed break in walk (9)
TREASURED : SURE(guaranteed;for certain) contained in(break in) TREAD(to walk).
5 Sort of cabbage and soya stew goes round 5 (5)
SAVOY : Anagram of(… stew) SOYA containing(goes round) V(Roman numeral for 5).

6 Powder coating’s used to protect furniture (4,5)
DUST COVER : DUST(powder;a substance in the form of fine particles) + COVER(an external coating).
7 Queen’s taking King’s gold — a mistake! (5)
ERROR : ER(abbrev. for “Elizabeth Regine”, the English queen) plus(…’s taking) R(abbrev. for “Rex”;king) plus(…’s;has) OR(the colour of gold in heraldry).
8 Criminal stole very quietly in and falls over (7)
TOPPLES : Anagram of(Criminal) STOLE contained in(… in) PP(abbrev. for “pianisimmo”, the musical instruction to play very quietly).
14 Article about Brook is riveting (9)
THRILLING : THING(an article not precisely described) containing(about) RILL(a brook;a small stream).
16 Out barking at the moon? (3,2,4)
NOT AT HOME : Anagram of(barking;mad) AT THE MOON.
17 Usage of a Dior tint’s transforming! (9)
TRADITION : Anagram of(…’s transforming) A DIOR TINT.
Defn: A habitual or customary practice.
18 Expert help’s required to go after flatfish (3,4)
DAB HAND : HAND(help, as in “to lend a hand”) placed below(…’s required to go after, in a down clue) DAB(a flatfish found mainly in the North Atlantic).
20 Hospital leader’s eyeing poor cleanliness (7)
HYGIENE : The 1st letter of(… leader) “Hospital” plus(…’s;has) anagram of(… poor) EYEING.
22 Part of Cambrian era’s turned up in field (5)
ARENA : Hidden in(Part of) reversal of(… turned up, in a down clue) “Cambrian era’s“.
Defn: … of, say, combat.
23 Iron a shirt, both sides, for banquet (5)
FEAST : FE(chemical symbol for the element, iron) + A + the leftmost and rightmost letters of(…, both sides) “shirt“.
24 Understand German artist’s power (5)
GRASP : G(abbrev. for “German”) + RA(abbrev. for Royal Academician, a member of the Royal Academy of Arts, an artist)‘S + P(symbol for “power” in physics).
Thanks Hectence and scchua. Good Quiptic, only DUST COVER a little weak
Thanks, Hectence and scchua. Package = deal passed me by, despite my working in the tourism industry … doh! The flatfish was also unknown – embarrassingly I only know the dab dance move! But everything was clearly clued (as always for Hectence) and hence no major issues completing.
Thank you Hectence and scchua.
Another bright spot, after doing the Cryptic, in a dull May Day here in France. AMERICANO for black coffee was new to me, I kept thinking of the magazine.
Many lovely smooth surfaces, I particularly liked the clues for EAVESDROP, AIMLESSLY and COMPASS.
I found this more difficult than the Chifonie but at least I finished this one!
Having lived in Italy, Americano for black coffee was familiar to me.
My favourite was NOT AT HOME
Thank you blogger and setter
Out barking at the moon? = NOT AT HOME is an excellent clue.
It works on at least three levels:
(1) the anagram
(2) if you’re not at home then you’re obviously out
(3) “barking at the moon” is a synonym for madness, as is “not at home”.
Maximum respect!
Belated thanks to Hectence and scchua.
I found this very chewy (it’s a compliment) and was glad to finish – although no write-in.
Absolutely loved DAB HAND. Mainly because I’m always looking to introduce into conversation the crossword expert who while no cook was, as s/he would explain, a “dab hand at fish fingers”. And now I have…
I have an Alphalpha proposal for the Quiptic: since it’s designed to inveigle aspirants towards the cryptic genre, why not put in all the crossers? Leaving the rest blank of course. That would make it no more/less difficult and would ease towards the parse rather than the solve, the biff(?), the plump, the downright hazarded guess.
I’m pretty sure that would make a more invitational puzzle for many.
You heard it here first…..