Rock in river covering snakelike fish (4)
I believe the answer is:
reel
'rock' is the definition.
(I've seen this before)
'river covering snakelike fish' is the wordplay.
'river' becomes 'r'.
'covering' means one lot of letters go next to another (on top of in down clue).
'snakelike 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 and angler's spool" , "Air" , "Film spool" , "Highland dance" , "Dance; winding cylinder" .)