Mark in small vehicle (4)
I believe the answer is:
scar
'mark' is the definition.
(synonyms)
'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'
'in' is the link.
(Other definitions for scar that I've seen before include "Bare craggy rock" , "Mark left by old wound" , "Trace of an old injury" , "nine [AL CAPONE] had to face" , "Effect of unpleasant experience" .)