Competitive driver, one in fancy car (5)
I believe the answer is:
racer
'competitive driver' is the definition.
(I've seen this in another clue)
'one in fancy car' is the wordplay.
'one' becomes 'ace' (ace has a value of one in some card games).
'in' is an insertion indicator.
'fancy car' becomes 'RR' (Rolls Royce).
'ace' inserted inside 'rr' is 'RACER'.
(Other definitions for racer that I've seen before include "sportsman" , "Speedway competitor, perhaps" , "Bike, perhaps" , "Fast-moving snake" , "One car" .)