Small vehicle for Mark (4)
I believe the answer is:
scar
'mark' is the definition.
(mark left by an injury)
'small vehicle' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'vehicle' becomes 'car' (car is a kind of vehicle).
's'+'car'='SCAR'
'for' acts as a link.
(Other definitions for scar that I've seen before include "Injury mark - cliff" , "nine [AL CAPONE] had to face" , "Healed-cut mark" , "Craggy rock formation" , "Blemish - cicatrix" .)