Athlete would be naughtier were I to enter (5)
I believe the answer is:
racer
'athlete' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
(Other definitions for racer that I've seen before include "One car" , "Bike, perhaps" , "Participant in speed contest" , "American snake" , "fast-moving vehicle" .)