Vehicle turns to leave (4)
I believe the answer is:
trap
'vehicle' is the definition.
(trap is a kind of vehicle)
'turns to leave' is the wordplay.
'turns' shows that the letters should be reversed in order.
'to leave' becomes 'part' (parting means leaving or separating).
'part' backwards is 'TRAP'.
(Other definitions for trap that I've seen before include "Snaring device" , "Ambush" , "Snare; light carriage" , "Snare; pony carriage" , "Part (anag.)" .)