Fish left in hat (7)
I believe the answer is:
bloater
'fish' is the definition.
(I know that bloater is a type of herring)
'left in hat' is the wordplay.
I cannot really understand how this works, but
'left' could be 'l' (common abbreviation) and 'l' is present in the answer.
'hat' could be 'boater' (boater is a kind of hat) and 'boater' is located in the remaining letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for bloater that I've seen before include "food" , "Buckling" , "Herring salted and smoked" , "Something fishy" , "Partially dried herring" .)