Boy eating beefburger ultimately becoming fat (4)
I believe the answer is:
lard
'fat' is the definition.
(lard is a kind of fat)
'boy eating beefburger ultimately' is the wordplay.
'boy' becomes 'lad' (lad is a kind of boy**).
'eating' indicates putting letters inside.
'ultimately' says to take the final letters.
The last letter of 'beefburger' is 'r'.
'lad' enclosing 'r' is 'LARD'.
'becoming' is the link.
(Other definitions for lard that I've seen before include "Clarified pig fat" , "Hard cooking fat" , "stuff with bacon" , "Pork fat" , "Animal fat" .)