Fat young man getting right inside (4)
I believe the answer is:
lard
'fat' is the definition.
(I know that lard is a type of edible fats and oils)
'young man getting right inside' is the wordplay.
'young man' becomes 'lad' (lad can mean a boy or young man).
'getting right' becomes 'r' (I can't explain this - if you can you should believe this answer much more).
'inside' means one lot of letters goes inside another.
'lad' going around 'r' is 'LARD'.
Can you help me to learn more?
(Other definitions for lard that I've seen before include "Animal fat" , "Pork fat" , "Soft white semisolid fat" , "F [FAT]" , "Fat Scottish landowner appears disheartened" .)