Freight doesn't start in Jason's craft (4)
I believe the answer is:
argo
'jason's craft' is the definition.
(I've seen this in another clue)
'freight doesn't start' is the wordplay.
'freight' becomes 'cargo' (synonyms).
'doesn't start' means to remove the first letter.
'cargo' with its initial letter taken off is 'ARGO'.
'in' acts as a link.
(Other definitions for argo that I've seen before include "Mythological ship" , "Jason's legendary ship" , "Transport to find the Golden Fleece" , "Agro about the constellation" , "Old ship" .)