Character marking spot by fire — it resists heat (5)
I believe the answer is:
pyrex
'it resists heat' is the definition.
(Pyrex is a heat-resistant material)
'character marking spot by fire' is the wordplay.
'character marking spot' becomes 'X' (X marks the spot).
'by' is a charade indicator (letters next to each other).
'fire' becomes 'pyre' (I have seen 'Funereal fire' mean 'pyre' so perhaps 'fire' could also mean 'pyre').
'x' after 'pyre' is 'PYREX'.
(Other definitions for pyrex that I've seen before include "Heat-resistant glass" , "Heat-proof ovenware" , "Heat-resistant glassware used in cooking" , "Heatproof glass" .)