Fat soldier catching fish (9)
I believe the answer is:
margarine
'fat' is the definition.
(thesaurus)
'soldier catching fish' is the wordplay.
'soldier' becomes 'marine' (I've seen this in other clues**).
'catching' means one lot of letters goes inside another.
'fish' becomes 'gar' (gar is a kind of fish).
'marine' going around 'gar' is 'MARGARINE'.
(Other definitions for margarine that I've seen before include "Reg and Marian make a butter substitute" , "F [FAT]" , "Is Ma angrier at this spread?" , "Fat-based butter substitute" , "Substitute spread for butter" .)