Boy eats last of leftover fat (4)
I believe the answer is:
lard
'fat' is the definition.
(lard is a kind of fat)
'boy eats last of leftover' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'eats' means one lot of letters goes inside another.
'last of' says to take the final letters.
The final letter of 'leftover' is 'r'.
'lad' enclosing 'r' is 'LARD'.
(Other definitions for lard that I've seen before include "Pig's fat" , "Pig fat used in cooking" , "F [FAT]" , "Aggrandise - embellish" , "Fat Scottish landowner appears disheartened" .)