Cars can turn in them (4)
I believe the answer is:
arcs
'turn in them' is the definition.
I can't judge whether this definition defines the answer.
'cars can' is the wordplay.
'can' indicates an anagram (I've seen 'canned' mean this (canned can mean drunk)).
'cars' is an anagram of 'ARCS'.
Can you help me to learn more?
(Other definitions for arcs that I've seen before include "Parts of a circle" , "Curves in cars" , "Segments of a circle" , "Streaks of electrical discharges" , "formations not straight" .)