(Please click here for this same blog but with a picture quiz added. Please do NOT post hereinbelow any comment relating to the picture quiz. Thank you.) Thanks Hypnos for an enjoyable, the last few bits giving a bit of pause. Definitions are underlined in the clues.
Across
1 Agreement announced for transatlantic transport (8)
CONCORDE : Homophone of(… announced) “concord”(agreement;harmony).
Defn: … capable of a top speed of over Mach 2.
5 Brush that’s about right for untidy figure (6)
SCRUFF : SCUFF(to brush;rub a surface, until it becomes scratched) containing(that’s about) R(abbrev. for “right”).
9 Dine fancily, taken with hot site for cooking in Dutch city (9)
EINDHOVEN : Anagram of(… fancily) DINE plus(taken with) H(abbrev. for “hot”) + OVEN(a site;place for cooking).
11 Number snubbed inventor – and another (5)
NOBEL : NO(abbrev. for “number”) + “Bell”(Alexander Graham, inventor of the telephone) minus its last letter(snubbed …).
Defn: … inventor, this time of dynamite.
12 Minister’s brass? (7)
STIPEND : Cryptic defn: Money;brass as fixed and regular payment to a minister of the clergy. As distinct from the tart;brass of a minister in government?
13 Tasteless stuff shown by, say, retired Catholic (7)
GELATIN : Reversal of(…, retired) EG(abbrev. for “exempli gratia”;for example;say) + LATIN(describing something that pertains to the Roman Catholic Church).
14 Noticed minor changes in investigators’ facility (8,4)
INCIDENT ROOM : Anagram of(… changes) NOTICED MINOR.
Defn: … in a police station.
18 Indulgence that’s not good getting approval (12)
RATIFICATION : “gratification”(indulgence;statisfying one’s appetites) minus(that’s not) “g”(abbrev. for “good”).
21 Plain promotional work that is following broadcast (7)
PRAIRIE : PR(abbrev. for “public relations”;promotional work) + [ IE(abbrev. for “id est”;that is) placed after(following, in an across clue) AIR(to broadcast over the radio or TV, say) ].
22 Unusually hardy conservationists in main outlet (7)
HYDRANT : Anagram of(Unusually) HARDY + NT(abbrev. for the National Trust for Places of Historic Interest or Natural Beauty, in full, the conservationist organisation in the UK).
Defn: An upright pipe serving as an outlet from the main pipe.
24 Sly feature in game clubs disowned (5)
RAMBO : “crambo”(game where one side must give a rhyme to a word or line provided by another) minus(… disowned) “c”(abbrev. for for “clubs”, the suit in a card game).
Defn: A feature film starring Sylvestor “Sly” Stallone.
25 Large Swedish company’s fashionable instantly (4,1,4)
LIKE A SHOT : L(abbrev. for “large”) + IKEA(the Swedish company famous for its ready-to-assemble furniture)‘S + HOT(fashionable;in).
26 Discussion of unduly solemn chap (6)
ERNEST : Homophone of(Discussion of) “earnest”(unduly solemn;serious, perhaps to the point of being zealous).
27 A boss set in order insulating material (8)
ASBESTOS : Anagram of(… in order) A BOSS SET.
Down
1 Notice left in city for rising comedian (6)
CLEESE : Reversal of(… for rising) [ SEE(to notice) + L(abbrev. for “left”) ] contained in(in) EC(the London postcode that includes almost all of the City of London) } .
Answer: John, of “Monty Python” and “Fawlty Towers” fame.
2 Sister, one kept in business as messenger (6)
NUNCIO : NUN(a sister in convent) + [ I(Roman numeral for “one”) contained in(kept in) CO(abbrev. for “company”, a business organisation)].
Defn: … sent by the Pope to a foreign capital as his diplomatic representative.
3 Shade from loch recorded (5)
OCHRE : Hidden in(from) “loch recorded “.
Defn: Any of the colours ranging from pale yellow to orange and red.
4 Article about college penned by French painter, philosopher and novelist (5,8)
DAVID MITCHELL : [ THE(the article in grammar) containing(about) C(abbrev. for “college”) ] contained in(penned by) [ DAVID(Jacque-Louis, French painter) + MILL(John Stuart, British philosopher) ].
Defn: An English novelist.
6 Artist embroidering tale in poetic form (9)
CANALETTO : Anagram of(embroidering) TALE contained in(in) CANTO(a main division of a long poem).
Answer: … from Venice.
7 Release top diplomat’s identity badge (8)
UNBUTTON : A country’s United Nations representative could be its top diplomat, and his/her identity badge could be called a U.N. button.
8 Line scrawled in note showing e.g. text on a storage device (8)
FILENAME : Anagram of(… scrawled) LINE contained in(in) FAME(note, as in “of note”;with fame).
Defn: What might be written on the outside of a file, a storage device for documents and such.
10 Go astern at sea to attract fish? It’s of scant value (2,5,6)
NO GREAT SHAKES : Anagram of(… at sea) GO ASTERN + HAKES(food fish with elongated bodies and large heads).
15 Criminal fellow restricted by tight pledges for repayment (9)
NEFARIOUS : F(abbrev. for “fellow”) contained in(restricted by) [ NEAR(tight;stingy) + IOUS(written pledges to pay off a debt, from “I owe you”) ].
16 A lot of fish served up in front of lake restaurant (8)
CREPERIE : Reversal of(… served up) “perch”(a freshwater food fish) minus its last letter(A lot of …) placed above(in front of) ERIE(one of the Great Lakes in N. America).
17 Figure on board supporting Labour politician, sham opponent (5,3)
STRAW MAN : MAN(a piece on a chess board) placed below(supporting) STRAW(Jack, British Labour politician).
Defn: A made-up, conveniently weak version of one’s argument or position presented by one’s opponent, so that the latter can easily tear it down.
19 Companion in seat crumpled bag (6)
SACHET : CH(abbrev. for a Companion of Honour, a title conferred by the sovereign of the British Commonwealth) contained in(in) anagram of(… crumpled) SEAT.
20 Rank Epstein’s work, say, not close to Moore (6)
STATUS : “statues”(examples of;say, British sculptor Jacob Epstein’s work) minus(not) the last letter of(close to) “Moore”. Henry Moore was another English sculptor, hence the surface reading.
Epstein and 
Moore – close or not close?
23 Noted bowler getting duck (5)
DRAKE : Double defn: 1st: Francis, Vice Admiral of the English fleet at that time, noted (in what is probably an apocryphal anecdote) to have finished his game of bowls even upon learning of the impending attack by the Spanish Armada; and 2nd: … of the male sex.
(Please do NOT post hereinbelow any comment relating to the picture quiz. Thank you.)
Thanks Hypnos and scchua.
I enjoyed the puzzle, especially NEFARIOUS and NO GREAT SHAKES, but failed to get RAMBO and DAVID MITCHELL until all the crossers were in.
Quite a toughie. I struggled with, among others, 7dn, thinking a top diplomat must be an ambassador, so that ‘HE’ (His/Her Excellency) had to be in the answer somewhere; imho the clue should have had a question mark at the end.
But thanks, Hypnos and scchua.