Incandescence (4)
(Other definitions for heat that I've seen before include "Toast" , "Energy that caues a rise in temperature" , "Preliminary race to qualify finalists" , "Hotness" , "eliminating round" .)
(Other definitions for fire that I've seen before include "Ready, aim . . . ." , "Sack; conflagration" , "There's no smoke without it" , "Flames; passion" , "... or combustion" .)
'incandescence' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for glow that I've seen before include "Emanate light" , "rosy colour" , "Gleam" , "Radiate or redden" , "Shine with warmth, emotion" .)