Fat boy eating last of sugar (4)
I believe the answer is:
lard
'fat' is the definition.
(lard is a kind of fat)
'boy eating last of sugar' is the wordplay.
'boy' becomes 'lad' (lad is a kind of boy**).
'eating' is an insertion indicator.
'last of' says to take the final letters.
The last letter of 'sugar' is 'r'.
'lad' placed around 'r' is 'LARD'.
(Other definitions for lard that I've seen before include "F [FAT]" , "Pork fat" , "Soft white semisolid fat" , "Pig's fat" , "stuff with bacon" .)