Coal (5)
I believe the answer is:
ember
'coal' is the definition.
(I know that coal can be written as ember)
This is the entire clue.
(Other definitions for ember that I've seen before include "Hot piece of wood or coal from a fire" , "what causes a warm glow" , "Burning coal" , "Glowing cinder" , "Glowing, smouldering piece of coal or wood" .)