Organic matter in soil (5)
I believe the answer is:
humus
'organic matter in soil' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for humus that I've seen before include "Decomposed vegetation" , "Rich organic material from dead leaves, plants etc" , "Organic matter in topsoil" , "Compost" , "Organic stuff" .)