The ground is dirty (4)
I believe the answer is:
soil
I believe this clue is a double definition.
'the ground' is the first definition.
(soil is a kind of ground)
'dirty' is the second definition.
('soil' can be a synonym of 'dirty')
'is' acts as a link.
(Other definitions for soil that I've seen before include "Earth; make dirty" , "Sully with earth" , "Earth for growing" , "Top layer of the surface of the earth" , "Loose upper layer of earth" .)