Ember (5)
(Other definitions for spark that I've seen before include "Fragment of a burning substance" , "Fiery particle" , "Tiny, red-hot particle" , "Means of ignition" , "Small electric flash" .)
'ember' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cinder that I've seen before include "Exhausted piece of coal" , "Refuse of burnt coal" , "Fragment left in the fireplace" , "Ember" , "Piece of burnt coal" .)