Boy has right to be fat (4)
I believe the answer is:
lard
'be fat' is the definition.
(I've seen this before)
'boy has right' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'has' is an insertion indicator.
'right' becomes 'r' (abbreviation).
'lad' enclosing 'r' is 'LARD'.
'to' is the link.
(Other definitions for lard that I've seen before include "Hard cooking fat" , "Pig fat used in cooking" , "Pig's fat" , "stuff with bacon" , "F [FAT]" .)