___ moss (4)
I believe the answer is:
peat
'moss' is the definition.
(peat moss is a kind of moss)
This is the entire clue.
(Other definitions for peat that I've seen before include "Fuel from marsh" , "Partially carbonised vegetable matter used as a fuel when dried" , "Decomposed vegetable matter" , "Form of fuel" , "Natural solid fuel" .)