Reason to squint (5)
I believe the answer is:
glare
'reason to squint' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for glare that I've seen before include "Angry stare" , "Shine intensely" , "Scowl angrily" , "Stare fiercely - 5 down (anag)" , "Too-bright light" .)