Piece of burnt coal (6)
I believe the answer is:
cinder
'piece of burnt coal' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for cinder that I've seen before include "Exhausted piece of coal" , "Ember" , "Incombustible matter left in the fireplace" , "Piece of burnt-out coal" , "Piece of burnt coal" .)