Rat coming from second vehicle (4)
I believe the answer is:
scab
'rat' is the definition.
('scab' can be a synonym of 'rat')
'second vehicle' is the wordplay.
'second' becomes 's' (common abbreviation).
'vehicle' becomes 'cab' (cab is a kind of vehicle**).
's'+'cab'='SCAB'
'coming from' acts as a link.
(Other definitions for scab that I've seen before include "part of healing process" , "one works during strike" , "Crust that forms on a healing skin wound" , "Crust of congealed blood" , "One continuing to work" .)