Fish left in hat (7)
I believe the answer is:
bloater
'fish' is the definition.
(bloater is a kind of fish)
'left in hat' is the wordplay.
I cannot quite see how this works, but
'left' could be 'l' (common abbreviation) and 'l' is found within the answer.
'hat' could be 'boater' (boater is a kind of hat) and 'boater' is present in the leftover 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" .)