Hit second vehicle (6)
I believe the answer is:
struck
'hit' is the definition.
(I've seen this before)
'second vehicle' is the wordplay.
'second' becomes 's' (common abbreviation).
'vehicle' becomes 'truck' (truck is a kind of vehicle).
's'+'truck'='STRUCK'
(Other definitions for struck that I've seen before include "hit hard" , "Thumped" , "dismantled" , "Hit as by lightning" , "Hit - withheld labour" .)