Soldier serving at sea must consume fish spread (9)
I believe the answer is:
margarine
'spread' is the definition.
(margarine is a spread)
'soldier serving at sea must consume fish' is the wordplay.
I cannot really see how this works, but
'at' could be 'in' (synonymous in some cases - eg at school, in school) and 'in' is found within the answer.
'soldier' could be 're' (Royal Engineer) and 're' is located in the remaining letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for margarine that I've seen before include "Reg and Marian make a butter substitute" , "Is Ma angrier at this spread?" , "F [FAT]" , "Vegetable fat" , "Item of food" .)