Meteorological effect caused by refraction (4)
I believe the answer is:
halo
'meteorological effect caused by refraction' 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 "Circle of (holy) light" , "that could make one light-headed?" , "Angelic aura" , "Angel's head-light" , "Ring (around sun or moon)" .)