Momentary flash of light (5)
I believe the answer is:
glint
'momentary flash of light' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for glint that I've seen before include "A very brief flash of light" , "Glittering flash" , "Flash of light (in the eye?)" , "Shine" , "Flicker of light" .)