Two for the Price of One by Viking

This puzzle introduced a novel variant of the "definition and letter mixture" (DLM) clue type wherein the jumble was actually of two related words with an extra letter. One of the words was defined in the clue and the other was to be entered in the grid. DLM clues are generally fairly easy to solve once one or two checked letters are known. Having only nine such clues to deal with meant that this was a fairly straightforward puzzle to solve. The pairs of words soon resolved as familiar pairs ("Kith and kin" was the first one Gregson unravelled). The extra letters were a jumble of HENDIADYS, a rhetorical figure in which two nouns are joined with "and". Gregson didn't feel the word pairs here exemplified this figure particularly well, or even at all, at least as far as the Chambers definition goes, but there was little doubt about the correct solutions.

Across
1 Signs around assorted costers' quarter? (13, two words)
MARKET CROSSES (Jumble COSTERS + E(ast) inside MARKS, & lit)
11 Base stuck in broker chair's more painful (6)
ACHIER (E (base) inside jumble CHAIR)
12 WaLK CYCLE BAck to sign (4)
BECK & CALL + Y
13 WoMEN SUFFER TO Achieve success (4)
FAME & FORTUNE + S
14 Attend or send bottle - OK? (7)
ENDORSE (Hidden in attEND OR SEnd)
16 Meanest support - run out for single (8)
FOOTIEST (FOOTREST with I replacing R)
17 Limitless colour in films of pollen (7)
INTINES (TINTINESS minus first and last letters)
19 Admit leaving with the current pusher (6)
Downriver without own (admit)
20 Listen to rock star exchanging line with fantastic amateur alto (10, four words)
GET A LOAD OF ((Bob) GELDOF with jumble A(mateur) ALTO replacing L(ine))
22 State is lUCKY NOTHING Ruins its image (4)
KING & COUNTRY + H
25 LocaL VIOLIN DUo have clear sound (4)
NULL & VOID + I
28 Eastern labels can wrongly read "washable" (10)
CLEANSABLE (Jumble E(astern) + LABELS CAN)
30 Short wig, half missing in fitting (6)
ABRUPT (BRUTUS minus half inside APT)
32 Cook a pizza leaving centre spread with meat and onions (7)
DOPIAZA (DO + jumble A PIZZA minus Z)
36 One's withdrawn alternative technology involving new routine (8)
ANCHORET (CHORE inside AT (alternative technology))
37 Mum's in Med hotspot to get burnt (7)
CREMATE (MA inside CRETE)
38 I THINK I'D Know about relationship (4)
KITH & KIN + D
39 THE DOME SHOUld provide fun for audience (4)
HOUSE & HOME + D
40 Chap in reversing old conveyance sticks in Kingussie (6)
CAMANS (MAN inside SAC rev.)
Yet another Scottish town puts in an appearance.
41 Paradoxically, it doesn't go by mail train (13, two words)
EXPRESS LETTER (Oblique definition)
A clue more suited to a broadsheet puzzle.

Down
1 Celebrate in high spirits - very loud American turns up with his bottle to pass round (7)
MAFFICK (FF + A(merican) rev. inside MICK)
2 Network secures short time for outside broadcast (6)
REMOTE (MO inside RETE)
3 Prickly heat treated with ice around midday (8)
ECHINATE (Jumble HEAT + ICE + N(oon))
4 Note seen in the previous clue (5)
THREE (RE (note) inside THE)
5 Date accepted for settlement in renewing entry? (7)
RENT-DAY (D(ate) + A(ccepted) inside jumble ENTRY)
6 Rule is to blame fOR RAW DEAL (5)
LAW & ORDER + A
7 Athenian lawman's caught one interrupting his singsong (7)
SCOLION (C(aught) and I (one) inside SOLON)
8 It's capital for Scot's chest? (4)
SARK (S (first letter of SCOT) + ARK, & lit.)
9 Artful, pincing gem, avoiding copper -- he's Dickensian (6)
SLEARY (PEAR (gem) minus P inside SLY)
10 Edmund's to learn about a small particle (4)
KAON (A inside KON)
15 No longer handle filter beverage for one (6)
STEALE (SILE with TEA replacing I)
18 Good SEASON DUE For fruit (5)
SAFE & SOUND + E
20 Mr Big's backing Australian rock (6)
GIBBER (REB (Mr) + BIG rev.)
21 Stop raising old tax, mostly counterfeit (8)
DIAPHONE (AID rev. + PHONE(y))
23 I'm more gloomy: joiner's retaining my money(7)
GLUMMER (M(y) + M(oney) inside GLUER)
24 Stick meat on skewers, mostly around sweet potatoes (7)
BATATAS (BAT + SATA(y) rev.)
26 I'm disgusted at role heroin plays (7)
LOATHER (Jumble AT ROLE + H(eroin)
27 District Engineer's tactless (6)
GAUCHE (GAU + CHE (engineer))
29 Latin American weasel climbing rope (6)
LARIAT (LA (Latin American) + TAIR rev.)
31 This uniQUE PIECE NATure left undisturbed (5)
PEACE & QUIET + N
33 (or 41?) Locate missing base involved (5)
OCTAL (Jumble LOCATE minus E)
41 is 33 in octal.
34 Letter provides small room (4)
ZETA (Two meanings)
35 Pot and cocaine confiscated from revolutionary politician (4)
HEMP (CHE minus C(ocaine) + MP)