Fall that impairs vision (8)
I believe the answer is:
cataract
This clue is a double definition.
'fall' is the first definition.
(I've seen this before)
'that impairs vision' is the second definition.
(I've seen this before)
(Other definitions for cataract that I've seen before include "Waterfall or eye ailment" , "big water surge" , "can obstruct your view" , "one may have causing blurred vision" , "Waterfall which might be seen in the eye" .)