Journey made by fish under river (4)
I believe the answer is:
ride
'journey' is the definition.
(ride is a kind of journey)
'fish under river' is the wordplay.
'fish' becomes 'ide' (I've seen this before**).
'under' means one lot of letters go next to another.
'river' becomes 'r'.
'ide' put after 'r' is 'RIDE'.
'made by' acts as a link.
(Other definitions for ride that I've seen before include "Travel (on a bike, perhaps)" , "Travel, on horseback, say" , "Sally" , "Travel on (a bicycle)" , "Be transported" .)