Deface small vehicle (4)
I believe the answer is:
scar
'deface' is the definition.
(I know this)
'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 "Remnant of wound" , "Skin mark; bare crag" , "Evidence of a past injury?" , "nine [AL CAPONE] had to face" , "Evidence of an old wound" .)