Fat boy? About right! (4)
I believe the answer is:
lard
'fat' is the definition.
(I know that lard is a type of edible fat)
'boy? about right' is the wordplay.
'boy?' becomes 'lad' (informal term for boy).
'about' is an insertion indicator.
'right' becomes 'r' (abbreviation).
'lad' going around 'r' is 'LARD'.
(Other definitions for lard that I've seen before include "Pig-fat" , "Pig fat used in cooking" , "Aggrandise - embellish" , "Soft white semisolid fat" , "F [FAT]" .)