Fish in sea spread (9)
I believe the answer is:
margarine
'spread' is the definition.
(margarine is a spread)
'fish in sea' is the wordplay.
'fish' becomes 'gar' (type of gar).
'in' is an insertion indicator.
'sea' becomes 'marine' (I've seen this before).
'gar' inserted into 'marine' is 'MARGARINE'.
(Other definitions for margarine that I've seen before include "Vegetable fat" , "Fat-based butter substitute" , "Reg and Marian make a butter substitute" , "Is Ma angrier at this spread?" , "Item of food" .)