Unhappy eating cold fish (4)
I believe the answer is:
scad
'fish' is the definition.
(scad is a kind of fish)
'unhappy eating cold' is the wordplay.
'unhappy' becomes 'sad' ('sad' can be a synonym of 'unhappy').
'eating' is an insertion indicator.
'cold' becomes 'c' (eg on taps).
'sad' enclosing 'c' is 'SCAD'.
(Other definitions for scad that I've seen before include "fish" , "A lot of money for American" , "Jack" .)