Swim in river with slippery fish (4)
I believe the answer is:
reel
'swim' is the definition.
(I've seen this before)
'river with slippery fish' is the wordplay.
'river' becomes 'r'.
'with' says to put letters next to each other.
'slippery fish' becomes 'eel' (I've seen this before**).
'r'+'eel'='REEL'
'in' acts as a link.
(Other definitions for reel that I've seen before include "Lively dance or fishing rod controller" , "A merry dance or an angler's winder" , "Drum - dance - music" , "Winding spool on a fishing rod" , "Dance; winding cylinder" .)