Luminous circle (4)
I believe the answer is:
halo
'luminous circle' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for halo that I've seen before include "Crown" , "Ring of light (signifying holiness)" , "that could make one light-headed?" , "Circle of light around the moon" , "Circle of (holy) light" .)