Mythical craft in a river journey (4)
I believe the answer is:
argo
'mythical craft' 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' (I've seen this in other clues).
'a'+'r'+'go'='ARGO'
'in' acts as a link.
(Other definitions for argo that I've seen before include "Jason's vessel" , "Mythical ship" , "Ancient ship" , "Stars once recognised in the south" , "Transport to find the Golden Fleece" .)