Sit on river and fish (4)
I believe the answer is:
ride
'sit' is the definition.
('ride' can be a synonym of 'sit')
'river and fish' is the wordplay.
'river' becomes 'r'.
'and' is a charade indicator (letters next to each other).
'fish' becomes 'ide' (I've seen this before).
'r'+'ide'='RIDE'
'on' is the link.
(Other definitions for ride that I've seen before include "Sally" , "badger" , "Go by horse" , "A dire sort of journey" , "Fairground attraction; go on horseback" .)