Freight to leave at the back of a vehicle (5)
I believe the answer is:
cargo
'freight' is the definition.
(synonyms)
'leave at the back of a vehicle' is the wordplay.
'leave' becomes 'go' (to go is to depart or leave).
'at the back of' means one lot of letters go next to another.
'a vehicle' becomes 'car' (car is a kind of vehicle).
'go' after 'car' is 'CARGO'.
'to' acts as a link.
(Other definitions for cargo that I've seen before include "Goods carried by vessel or vehicle" , "Payload" , "Freight on board ship" , "Freight of ship, plane or truck" , "Shipment" .)