Gloomy, having eaten hard fish (4)
I believe the answer is:
shad
'fish' is the definition.
(shad is a kind of fish)
'gloomy having eaten hard' is the wordplay.
'gloomy' becomes 'sad' ('sad' can be similar in meaning to 'gloomy').
'having eaten' indicates putting letters inside.
'hard' becomes 'h'.
'sad' placed around 'h' is 'SHAD'.
(Other definitions for shad that I've seen before include "Fish; dash (anag.)" , "fish that's been caught" , "Fish related to the herring" , "Herring-like fish" .)