Champion in car, a speed merchant (5)
I believe the answer is:
racer
'a speed merchant' is the definition.
(someone who drives fast)
'champion in car' is the wordplay.
'champion' becomes 'ace' (both can mean a person excelling at something).
'in' indicates putting letters inside.
'car' becomes 'RR' (Rolls Royce).
'ace' going into 'rr' is 'RACER'.
(Other definitions for racer that I've seen before include "Greyhound" , "Bike, perhaps" , "Athlete" , "Fast competitive car or animal" , "Participant in speed contest" .)