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