A vehicle to travel as freight (5)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
cargo
'freight' is the definition.
(synonyms)
'a vehicle to travel' is the wordplay.
'a 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 "stuff to be carried by ship" , "Goods in transit" , "Freight, load" , "Goods carried by vessel or vehicle" , "Freight of ship, plane or truck" .)