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' is an insertion indicator.
'fish' becomes 'gar' (type of fish).
'marine' placed around '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" .)