Small vehicle making a mark (4)
I believe the answer is:
scar
'a 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'
'making' is the link.
(Other definitions for scar that I've seen before include "Craggy outcrop" , "Steep cliff; skin mark" , "Steep rock - blemish" , "Mark left by the healing of injured tissue" , "Old-wound mark" .)