The freight, try to get onto the vehicle (5)
I believe the answer is:
cargo
'the freight' is the definition.
('cargo' can be a synonym of 'freight')
'get onto the vehicle' is the wordplay.
'get' becomes 'go' (synonyms).
'onto' is a charade indicator (letters next to each other).
'the vehicle' becomes 'car' (car is a kind of vehicle).
'go' put after 'car' is 'CARGO'.
'try to' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for cargo that I've seen before include "Goods in transit" , "What gets carried" , "Freight of ship, plane or truck" , "Payload" , "Load carried" .)