Replacement part for a vehicle (4)
I believe the answer is:
trap
'a vehicle' is the definition.
(trap is a kind of vehicle)
'replacement part' is the wordplay.
'replacement' is an anagram indicator.
'part' anagrammed gives 'TRAP'.
'for' is the link.
(Other definitions for trap that I've seen before include "Confine" , "Pitfall" , "Light carriage - it's sometimes sprung!" , "Means of transport - sometimes sprung!" , "corner" .)