Freight unopened in Jason's ship (4)
I believe the answer is:
argo
'jason's ship' is the definition.
(I've seen this before)
'freight unopened' is the wordplay.
'freight' becomes 'cargo' ('cargo' can be a synonym of 'freight').
'unopened' means to remove the first letter.
'cargo' with its first letter taken off is 'ARGO'.
'in' acts as a link.
(Other definitions for argo that I've seen before include "Mythological ship" , "Old boat" , "Old ship" , "ancient boat" , "Jason's craft" .)