Fat fellow about right (4)
I believe the answer is:
lard
'fat' is the definition.
(lard is a kind of fat)
'fellow about right' is the wordplay.
'fellow' becomes 'lad' (I have seen 'Young fellow' mean 'lad' so perhaps 'fellow' could also mean 'lad'**).
'about' indicates putting letters inside.
'right' becomes 'r' (common abbreviation).
'lad' going around 'r' is 'LARD'.
(Other definitions for lard that I've seen before include "Pig fat used in cooking" , "Pig-fat" , "Animal fat" , "stuff with bacon" , "F [FAT]" .)