Momentary flash of light (5)
I believe the answer is:
glint
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">momentary flash of light</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)