Rock in river with fish underneath (4)
I believe the answer is:
reel
'rock' is the definition.
(reel can mean to rock around)
'river with fish underneath' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'with' is a charade indicator (letters next to each other).
'fish underneath' becomes 'eel' (eel is a type of fish. I am not sure about the 'underneath' bit.).
'r'+'eel'='REEL'
'in' is the link.
Can you help me to learn more?
(Other definitions for reel that I've seen before include "Cylinder on which something is wound" , "Lively dance; bobbin" , "Dance; winding drum" , "spin" , "Stagger in Irish dance" .)