Load vehicle then leave (5)
I believe the answer is:
cargo
'load' is the definition.
(I know that load can be written as cargo)
'vehicle then leave' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'then' means one lot of letters go next to another.
'leave' becomes 'go' (to go is to depart or leave).
'car'+'go'='CARGO'
(Other definitions for cargo that I've seen before include "Goods carried on ship or plane" , "Load carried" , "Consignment" , "Goods being transported" , "Freight on board ship" .)