Fat gets right inside boy (4)
I believe the answer is:
lard
'fat' is the definition.
(I know that lard is a type of edible fat)
'right inside boy' is the wordplay.
'right' becomes 'r' (common abbreviation).
'inside' is an insertion indicator.
'boy' becomes 'lad' (informal term for boy).
'r' placed inside 'lad' is 'LARD'.
'gets' is the link.
(Other definitions for lard that I've seen before include "White cooking fat" , "Fat Scottish landowner appears disheartened" , "Aggrandise - embellish" , "Pork fat" , "Pig-fat" .)