Defile the earth (4)
I believe the answer is:
soil
I believe this clue is a double definition.
'defile' is the first definition.
(I've seen this in another clue)
'the earth' is the second definition.
(earth can mean soil or dirt)
(Other definitions for soil that I've seen before include "Dirty - dirt" , "Loose upper layer of earth" , "Sully with earth" , "Top layer of the surface of the earth" , "Earth; stain" .)