The dying residue of a fire (6)
I believe the answer is:
embers
'the dying residue of a fire' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for embers that I've seen before include "Dying residue of a fire" , "Dying coals in fire" , "Vestiges of fire" , "Live coals" , "Remains of coal fire" .)