Top layer of the surface of the earth (4)
I believe the answer is:
soil
'top layer of the surface of the earth' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for soil that I've seen before include "Dirty - dirt" , "Earth; stain" , "In which plants grow" , "Earth for growing" , "Loose upper layer of earth" .)