Thanks to Poins for an enjoyable puzzle. Definitions are underlined in the clues.
Across
1 Against stopping revolutionary comrade’s vehicular procession (9)
MOTORCADE : TO(in contact with/against, as in “the gun was put to his head…”) contained in(stopping) anagram of(revolutionary) COMRADE.
6 Greatly reduce rent (5)
SLASH : Double defn: 1st: … as with a selling price.
9 No longer effective without king becoming an object of fear (5)
DREAD : DEAD(no longer effective/functioning) containing(without) R(abbrev. for “Rex”/king).
10 Country starts to impose crippling austerity after Castro’s broadcast (5,4)
COSTA RICA : 1st letters, respectively, of(starts to) “impose crippling austerity” placed after(after) anagram of(…’s broadcast) CASTRO.
11 Automatic for Bottom perhaps (10)
MECHANICAL : Double defn: 2nd: Characters in Shakespeare’s “A Midsummer Night’s Dream”, named for their occupations as skilled manual labourers, of which Bottom is an example/perhaps.
12 The French newspaper inclined towards socialism (4)
LEFT : LE(French for the article “the”) + FT(abbrev. for the newspaper, the Financial Times).
Defn: …, on the left side of the political spectrum, traditionally.
14 Novelist found in dingy island retreat for the most part (7)
DRABBLE : DRAB(dingy/gloomy) + reversal of(… retreat) “Elba”(the Mediterranean island) minus its last letter(for the most part).
Answer: Dame Margaret or her father, John, both English novelists.
15 Hesitation before Jew needing no introduction becomes a recluse (7)
EREMITE : ER(an expression of hesitation) plus(before) “Semite”(a member of the peoples who speak or spoke a Semitic language, including the Jews and Arabs) minus its 1st letter(needing no introduction).
17 One for example discovered in oddly neglected hedge in Bury (7)
INTEGER : 1st, 3rd and 5th letters deleted from(oddly neglected) “hedge” contained in(in) INTER(to bury a corpse).
Defn: The number “One” is an example of it.
19 Virginia looking into good health in retirement exhibited by southern Europeans (7)
SLOVAKS : [ VA(abbrev. for the US state of Virginia) contained in(looking into) reversal of(… in retirement) [ SKOL(originally Scandinavian, a drinking toast wishing one good health) ] placed after(exhibited by) S(abbrev. for “southern”).
Defn: … from Slovakia, of course.
20 Means to go west on large boat (4)
YAWL : Reversal of(… to go west, in an across clue) WAY(a means of doing something/a procedure) plus(on) L(abbrev. for “large”).
22 Crucial as Poins is in effect loveless (10)
IMPERATIVE : I’M(Poins is/I am, the setter using the self-referential first person pronoun) + “operative”(in effect, as in “the new rules are operative from next month”) minus(…less) “o”(letter signifying 0/love in tennis scores).
25 Collective anger essentially admitted by chief in time (9)
AGGREGATE : [ Middle letter of(… essentially) “anger” contained in(admitted by) GREAT(chief/the principal part or person, as can be found in “___ the Great”) ] contained in(in) AGE(a period of time).
26 That’s assuming grandma is unimaginative (5)
INANE : IE(abbrev. for “id est”/that is) containing(assuming) NAN(a familiar term for one’s grandma).
27 Old Penny Black is small and neat (5)
DINKY : D(symbol for the old UK currency, the penny) + INKY(black as black ink).
28 Most sympathetic loan provider initially ignored during trial (9)
TENDEREST : “lender”(a loan provider) minus its 1st letter(initially ignored) contained in(during) TEST(a trial/an experiment).
Down
1 A precocious little girl whichever way you look at it (5)
MADAM : A palindrome(… whichever way you look at it).
Defn: …, especially one who’s conceited or bossy.
2 Distinctly defined charm associated with Turkey (9)
TRENCHANT : ENCHANT(to charm/to fill with delight) placed below(associated with, in a down clue) TR(the International country code for Turkey).
Defn: …/clear-cut.
3 Daughter entering about to steal something to eat (3,7)
RED CABBAGE : D(abbrev. for “daughter”) contained in(entering) [ RE(with reference to/about) + CABBAGE(to steal/pilfer) ].
4 Foolishly crave owning most of his record collection (7)
ARCHIVE : Anagram of(Foolishly) CRAVE containing(owning) 1st 2 letters of(most of) “his“.
5 Catch WWII performers entertaining new soldiers (7)
ENSNARE : ENSA(abbrev. for the Entertainments National Service Association, an organisation that provided entertainment to Word War II British military personnel/WWII performers) containing(entertaining) N(abbrev. for “new”) + RE(abbrev. for “Royal Engineers”, a corps in the British Army/soldiers). Nice surface.
6 Witch doctor dismissing article as false (4)
SHAM : “shaman”(a witch doctor/ among some peoples, one regarded as having access to good and evil spirits) minus(dismissing) “an”(a grammatical article).
7 Become apparent in some of Vasari’s eccentricities (5)
ARISE : Hidden in(some of) “Vasari’s eccentricities“.
8 Callous as Doris would become after Rex left (9)
HEARTLESS : How to describe(as … would become) the word “Doris” after its central letter/heart “r”(abbrev. for “Rex”/king) is deleted(left).
13 Throw into confusion with failure to capture gunmen in the centre of Honolulu (10)
DEMORALISE : DEMISE(the end of/the failure of an enterprise or institution) containing(to capture) [ RA(abbrev. for the Royal Artillery, the Gunners in the British Army) contained in(in) middle 2 letters of(the centre of) “Honolulu” ].
14 Performed gripping song by girl about old farm worker (9)
DAIRYMAID : DID(performed a deed) containing(gripping) [ AIR(a song/tune) plus(by) reversal of(… about) AMY(a feminine name) ].
16 At home getting a couple of texts after a former president turned up dead (9)
INANIMATE : IN(a home/not out and about) plus(getting) { 1st 2 letters of(couple of) “texts” placed below(after, in a down clue) [ A + reversal of(… turned) AMIN(Idi, former president and dictator of Uganda) ] }.
18 Unrestrained bombast overwhelming American politician (7)
RAMPANT : RANT(bombast/a verbal onslaught) containing(overwhelming) [ A(abbrev. for “American”) + MP(abbrev. for a Member of Parliament/an elected politician) ].
19 Be seen assembled outside home in which spirits may be found (7)
SHEBEEN : Anagram of(… assembled) BE SEEN containing(outside) H(abbrev. for “home”, as in, say, football league tables).
Defn: …, liquid spirits, that is, in an unlicensed establishment.
21 Going after valuables originally missing from vehicle (5)
WAGON : ON(going/functioning, as in “the washing machine is still on”) placed below(after, in a down clue) “swag”(valuables/stolen goods) minus its 1st letter(originally missing).
23 Make a strenuous effort to disregard hint of presumption in scientific witness (5)
EXERT : 1st letter of(hint of) “presumption” deleted from(to disregard … in) “expert”(a witness who is systematic and methodical/scientific)
24 Heroism without support? Absolutely (4)
VERY : “bravery”(heroism) minus(without) “bra”(brassiere/support for the feminine breast).
Defn: …/completely.
Yes, enjoyable. A number of answers suggested themselves but the wordplay took a bit of assembling. 3d stumped me. Chambers has ‘cabbage’ as a verb marked as archaic slang. I thought I was pretty archaic but i’d never come across that meaning. Thanks to Scchua and Poins.
Re ‘cabbage’: me neither (aged 71).
Snap
We didn’t know ‘cabbage’ for ‘steal’ either, but wondered if it might be rhyming slang, possibly ‘cabbage patch’ for ‘snatch’, so we were quite surprised to find it as a separate entry of “uncertain origin” in Chambers.
We were tempted to put ‘cavalcade’ for 1ac, thinking ‘against’ might indicate V[ersus] and the ‘revolutionary’ might be Cade, the 15th century rebel but as the rest wouldn’t parse we soon saw the right answer. The other clue that gave us trouble was 19ac where we thought the ‘va’ was part of the reversal and we were trying to find a word involving ‘Slav’; only when we got 13 and 16 dn did we see the obvious.
Otherwise it was all pretty straightforward, though still something of a challenge. Our only other observation, in reference to 24dn, is that ‘supporter’ for ‘bra’ seems to be becoming a bit of a cliché in crosswordland these days.
Favourites were MECHANICAL and DINKY. Thanks, Poins and scchua.