Fat boy eating last of burger (4)
I believe the answer is:
lard
'fat' is the definition.
(I know that lard is a type of edible fats and oils)
'boy eating last of burger' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'eating' is an insertion indicator.
'last of' suggests the final letters.
The last letter of 'burger' is 'r'.
'lad' going around 'r' is 'LARD'.
(Other definitions for lard that I've seen before include "White cooking fat" , "Hard cooking fat" , "Clarified pig fat" , "Pig's fat" , "Aggrandise - embellish" .)