Highest part of the heavens, thought by the ancients to contain the pure element of fire (8)
I believe the answer is:
empyrean
(Other definitions for empyrean that I've seen before include "Rap enemy under celestial vault" , "The heavens (poetic)" , "vault of the sky, celestial heaven" , "Heavenly (archaic)" , "May preen about the celestial vault" .)