Leatherneck consuming fish spread (9)
I believe the answer is:
margarine
'spread' is the definition.
(margarine is a spread)
'leatherneck consuming fish' is the wordplay.
'leatherneck' becomes 'marine' (synonyms).
'consuming' indicates putting letters inside.
'fish' becomes 'gar' (type of fish).
'marine' enclosing 'gar' is 'MARGARINE'.
(Other definitions for margarine that I've seen before include "Is Ma angrier at this spread?" , "Substitute spread for butter" , "F [FAT]" , "Vegetable fat" , "Fat-based butter substitute" .)