Rotter having success in Home Counties (5)
I believe the answer is:
swine
'rotter' is the definition.
(both can mean a horrible person)
'success in home counties' is the wordplay.
'success' becomes 'win'.
'in' is an insertion indicator.
'home counties' becomes 'se' (Home Counties in the South East of England).
'win' put within 'se' is 'SWINE'.
'having' is the link.
(Other definitions for swine that I've seen before include "cad" , "Pig; unpleasant person" , "Rotter" , "See 9" , "such a difficult thing" .)