Second vehicle for Mark (4)
I believe the answer is:
scar
'for mark' is the definition.
(mark left by an injury)
'second vehicle' is the wordplay.
'second' becomes 's' (common abbreviation).
'vehicle' becomes 'car' (car is a kind of vehicle).
's'+'car'='SCAR'
(Other definitions for scar that I've seen before include "Mark of a wound remaining on the skin" , "reef" , "brand" , "Remnant of wound" , "Effect of unpleasant experience" .)