Swim with river fish (4)
I believe the answer is:
reel
'swim' is the definition.
(I've seen this before)
'with river fish' is the wordplay.
'with' says to put letters next to each other.
'river' becomes 'r' (abbreviation for river).
'fish' becomes 'eel' (eel is a kind of fish).
'r' put next to 'eel' is 'REEL'.
(Other definitions for reel that I've seen before include "Whirl around" , "Something to fish with" , "Lively dance; bobbin" , "Stagger around in the dance" , "Dance - swim" .)