Method: Solve the clues and fit them where they will go.

 

A One car endlesly needs repair to exhaust – it's causing pollution (4,4)

B Hollow building block mostly made of B? (5)

C Occasional party had by temporary staff (6,6); Press, if touring Ross, would discover what you're doing (5)

D One could go on a spin, being relatively sober (5)

E The King is leaving Queen – that's fishy! (5)

F Courage brew mostly fresh ales with a head (12)

G Groups gain eminence with this producer (9)

H Greet Eastern Rastafarian leader familiarly (5)

I Within the Latin translation of... (5)

J ...1. John, "I relent" wrongly becomes "I require" (Pilate, in speech) (8)

K They're up for a party (5)

L Flash sort of conductor (9)

M Associatin' with unpleasant person and causing trouble (6,2); Matchstick finally having gone out, possibly making fire with this? (9)

N The blackness of inferior rubber? (9)

O Where the French set off by boat (9)

P The Puppy of the Baskervilles (1915) and The Young Man and the Sea (1955)? (8)

Q Live fast (5)

R Steps taken by bell-hop? (4-5)

S What MPs in the House would have been for Guy Fawkes (7,7)

T Incredibly, one can teach without a student in this college (9)

U Let go one French worker (6)

V Destructive potato blight finally eradicated with injection of nitrogen? (9)

W Voiced question showing unfamiliarity with sewing kit – thus making a bloomer (7,7)

X Get cross with man who's a believer (4)

Y The old article by Lamb (4)

Z Last ball gets hit back by English opener? That should be "belted" (6)