Soldier at sea consuming fish paste (9)
I believe the answer is:
margarine
'paste' is the definition.
(I know that margarine is a type of paste)
'soldier at sea consuming fish' is the wordplay.
'soldier at sea' becomes 'marine' (I've seen this before).
'consuming' indicates putting letters inside.
'fish' becomes 'gar' (type of gar).
'marine' placed around 'gar' is 'MARGARINE'.
(Other definitions for margarine that I've seen before include "F [FAT]" , "Item of food" , "Fat-based butter substitute" , "Vegetable fat" , "Is Ma angrier at this spread?" .)