Fat boy eating last of butter (4)
I believe the answer is:
lard
'fat' is the definition.
(lard is a kind of fat)
'boy eating last of butter' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'eating' is an insertion indicator.
'last of' indicates one should take the final letters.
The last letter of 'butter' is 'r'.
'lad' going around 'r' is 'LARD'.
(Other definitions for lard that I've seen before include "Clarified pig fat" , "Pig-fat" , "Fat Scottish landowner appears disheartened" , "F [FAT]" , "Pig's fat" .)