The load to leave on a vehicle? (5)
I believe the answer is:
cargo
'the load' is the definition.
('cargo' can be a synonym of 'load')
'leave on a vehicle?' is the wordplay.
'leave' becomes 'go'.
'on' says to put letters next to each other.
'a vehicle?' becomes 'car' (car is a kind of vehicle).
'go' after 'car' is 'CARGO'.
'to' is the link.
(Other definitions for cargo that I've seen before include "Freight on board ship" , "Hold contents" , "Goods in transit" , "Goods carried by vessel or vehicle" , "Freight of ship, plane or truck" .)