Swim in river with fish (4)
I believe the answer is:
reel
'swim' is the definition.
(I've seen this before)
'river with fish' is the wordplay.
'river' becomes 'r'.
'with' means one lot of letters go next to another.
'fish' becomes 'eel' (eel is a kind of fish**).
'r'+'eel'='REEL'
'in' is the link.
(Other definitions for reel that I've seen before include "unwind?" , "Stagger in Irish dance" , "Dance; winding drum" , "Scottish/Irish dance" , "Folk dance" .)