Vehicle to travel as freight (5)
I believe the answer is:
cargo
'freight' is the definition.
(synonyms)
'vehicle to travel' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'to travel' becomes 'go' (synonyms).
'car'+'go'='CARGO'
'as' is the link.
(Other definitions for cargo that I've seen before include "Ship's load" , "Stuff on board" , "Goods carried by ship, aircraft" , "Ship's freight" , "Goods in transit" .)