Freight to travel on passenger transport (5)
I believe the answer is:
cargo
'freight' is the definition.
(synonyms)
'travel on passenger transport' is the wordplay.
I cannot quite understand how this works, but
'travel' could be 'go' (synonyms) and 'go' is located in the answer.
'transport' could be 'car' (car is a method of transport) and 'car' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for cargo that I've seen before include "Shipment" , "Ship's load" , "Baggage" , "Freight on board ship" , "Ship's freight" .)