Fat boy right to tuck in (4)
I believe the answer is:
lard
'fat' is the definition.
(I know that lard is a type of edible fats and oils)
'boy right to tuck in' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'right' becomes 'r' (common abbreviation).
'to tuck in' means one lot of letters goes inside another.
'lad' going around 'r' is 'LARD'.
(Other definitions for lard that I've seen before include "Soft white semisolid fat" , "White cooking fat" , "stuff with bacon" , "Aggrandise - embellish" , "Hard cooking fat" .)