River fish potentially found in rocks (5)
I believe the answer is:
reels
'rocks' is the definition.
'river fish' is the wordplay.
'river' becomes 'r'.
'fish' becomes 'eels' (I've seen this before).
'r'+'eels'='REELS'
'potentially found in' acts as a link.
(Other definitions for reels that I've seen before include "performed by dancers" , "Spools" , "Staggers about in Irish dances" , "Staggers about in lively dances" , "Scottish dances" .)