Fat lad going round with a redhead (4)
I believe the answer is:
lard
'fat' is the definition.
(lard is a kind of fat)
'lad going round with a redhead' is the wordplay.
'going round with' indicates anagramming the letters.
'a redhead' becomes 'r' (the head/first letter of 'red').
'lad' mixed up with 'r' makes 'LARD'.
(Other definitions for lard that I've seen before include "F [FAT]" , "Pork fat" , "stuff with bacon" , "Animal fat" , "Clarified pig fat" .)