Jason's boat makes a river journey (4)
I believe the answer is:
argo
'jason's boat' is the definition.
(I've seen this in another clue)
'a river journey' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'journey' becomes 'go' (journey means to go or travel).
'a'+'r'+'go'='ARGO'
'makes' acts as a link.
(Other definitions for argo that I've seen before include "Name of ship sailed by Jason in search of the Golden Fleece" , "Ship in which Jason searched for the Golden Fleece" , "Fabulous ship" , "Mythical ship" , "ancient boat" .)