Cars go round the curves (4)
I believe the answer is:
arcs
'the curves' is the definition.
(arc is a kind of curve)
'cars go round' is the wordplay.
'go round' indicates an anagram.
'cars' with letters rearranged gives 'ARCS'.
(Other definitions for arcs that I've seen before include "Curving trajectories" , "formations not straight" , "Portions of a circle" , "Streaks of electrical discharges" , "Parts of a circle" .)