One getting into expensive car to compete (5)
I believe the answer is:
racer
'one getting into expensive car to compete' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'one getting into expensive car' is the wordplay.
'one' becomes 'ace' (synonyms).
'getting into' indicates putting letters inside.
'expensive car' becomes 'rr' (abbreviation for Rolls Royce).
'ace' placed inside 'rr' is 'RACER'.
(Other definitions for racer that I've seen before include "someone driving very fast?" , "Fast competitive car or animal" , "Speedway competitor, perhaps" , "Fast-moving animal" , "Bike, perhaps" .)