Fish sound river margin (4)
I believe the answer is:
brim
'river margin' is the definition.
'brim' can be an answer for 'margin' (I've seen this in another clue). I am unsure of the 'river' bit.
'fish sound' is the wordplay.
'fish' becomes 'bream' (bream is a kind of fish).
'sound' indicates a 'sounds like' (homophone) clue.
'bream' sounds like 'BRIM'.
Can you help me to learn more?
(Other definitions for brim that I've seen before include "Projecting edge of hat" , "Be full; edge of hat" , "Lip; part of 6 Down" , "Edge (of cup or hat)" , "Lip of bowl, cup etc" .)