Dirty ground (4)
I believe the answer is:
soil
I believe this clue is a double definition.
'dirty' is the first definition.
('soil' can be a synonym of 'dirty')
'ground' is the second definition.
(soil is a kind of ground)
(Other definitions for soil that I've seen before include "Sully with earth" , "Loose upper layer of earth" , "Earth; stain" , "In which plants grow" , "refuse" .)