Fat boy taking run (4)
I believe the answer is:
lard
'fat' is the definition.
(lard is a kind of fat)
'boy taking run' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'taking' indicates putting letters inside.
'run' becomes 'r' (cricket abbreviation).
'lad' enclosing 'r' is 'LARD'.
(Other definitions for lard that I've seen before include "stuff with bacon" , "Animal fat" , "Pig fat used in cooking" , "Soft white semisolid fat" , "White cooking fat" .)