Freight vehicle set off (5)
I believe the answer is:
cargo
'freight' is the definition.
(synonyms)
'vehicle set off' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'set off' becomes 'go' (I've seen this in other clues).
'car'+'go'='CARGO'
(Other definitions for cargo that I've seen before include "Goods carried by a large vehicle" , "stuff to be carried by ship" , "Freight, load" , "Goods being transported" , "Freight of ship, plane or truck" .)