Harold only has light (4)
I believe the answer is:
halo
'light' is the definition.
(I know that halo is a type of light)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for halo that I've seen before include "Nimbus" , "that could make one light-headed?" , "Circle of light round the sun or moon" , "Ring of light (signifying holiness)" , "Luminous ring" .)