Halfwit left stuffing fish (4)
I believe the answer is:
clod
'halfwit' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'left stuffing fish' is the wordplay.
'left' becomes 'l' (common abbreviation).
'stuffing' indicates putting letters inside.
'fish' becomes 'cod' (cod is a kind of fish).
'l' inserted inside 'cod' is 'CLOD'.
Can you help me to learn more?
(Other definitions for clod that I've seen before include "Lump of earth - stupid person" , "Lump of earth - nitwit" , "Piece of earth" , "Compact mass - stupid person" , "dope?" .)