Fireball, that which illuminates northern parts (9)
I believe the answer is:
lightning
'fireball' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for lightning that I've seen before include "Electric discharge in the atmosphere" , "Precaution against electrical storms" , "Metal device on building for diverting natural electric discharges" , "Flash in the sky" , "Natural electrical discharge" .)