Mark's small vehicle (4)
I believe the answer is:
scar
'mark's' 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'
(Other definitions for scar that I've seen before include "Mark of former injury" , "Old wound - bare cliff" , "Evidence of a past injury?" , "Mark left on healed tissue" , "Bare rocky cliff - injury" .)