Fat boy eating another, ultimately (4)
I believe the answer is:
lard
'fat' is the definition.
(lard is a kind of fat)
'boy eating another ultimately' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'eating' is an insertion indicator.
'ultimately' suggests the final letters.
The final letter of 'another' is 'r'.
'lad' going around 'r' is 'LARD'.
(Other definitions for lard that I've seen before include "F [FAT]" , "Pig's fat" , "Hard cooking fat" , "Fat Scottish landowner appears disheartened" , "Aggrandise - embellish" .)