Fish's bitter partner (7)
I believe the answer is:
alewife
'fish's' is the definition.
(alewife is a kind of fish)
'bitter partner' is the wordplay.
'bitter' becomes 'ale' ('ale' can be a synonym of 'bitter').
'partner' becomes 'wife' (wife is a kind of partner).
'ale'+'wife'='ALEWIFE'
(Other definitions for alewife that I've seen before include "a catch" , "serving fish" , "superior, > swimmer found" .)