Fish lake wearing straw hat (7)
I believe the answer is:
bloater
'fish' is the definition.
(bloater is a kind of fish)
'lake wearing straw hat' is the wordplay.
'lake' becomes 'l'.
'wearing' means one lot of letters goes inside another.
'straw hat' becomes 'boater' ('boater' can be a synonym of 'straw hat').
'l' inserted into 'boater' is 'BLOATER'.
(Other definitions for bloater that I've seen before include "Buckling" , "food" , "Partially dried herring" , "Herring salted and smoked" , "that will more than fill one up" .)