Mark in second vehicle (4)
I believe the answer is:
scar
'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'
'in' acts as a link.
(Other definitions for scar that I've seen before include "Mark of healed cut" , "Cicatrice" , "Sign of old injury" , "Mark on body or tree" , "Mark left on healed tissue" .)