Boy has right to get fat (4)
I believe the answer is:
lard
'fat' is the definition.
(I know that lard is a type of edible fat)
'boy has right' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'has' means one lot of letters goes inside another.
'right' becomes 'r' (common abbreviation).
'lad' going around 'r' is 'LARD'.
'to get' acts as a link.
(Other definitions for lard that I've seen before include "stuff with bacon" , "White cooking fat" , "Pig-fat" , "Pig's fat" , "Pork fat" .)