Luminous ring (4)
I believe the answer is:
halo
'luminous ring' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for halo that I've seen before include "Circle over a saint" , "Ring of light (signifying holiness)" , "Imaginary circle over saint's head" , "Ring (around sun or moon)" , "Aura representing saintliness" .)