Slovenly woman is scum (4)
I believe the answer is:
slag
This clue is a double definition.
'slovenly woman' is the first definition.
'scum' is the second definition.
(slag is a kind of scum)
'is' is the link.
(Other definitions for slag that I've seen before include "Perfect" , "Mound of waste matter from mine" , "dissolute man (more recently)" , "Scum on metal - dissolute woman" , "Coal mining waste" .)