A drop of water (8)
I believe the answer is:
cataract
This clue is a double definition.
'a drop' is the first definition.
(I've seen this before)
'water' is the second definition.
(I know that cataract is a type of waterfall)
'of' is the link.
(Other definitions for cataract that I've seen before include "can obstruct your view" , "Waterfall and eye defect" , "Waterfall and eye disease" , "one may have causing blurred vision" , "Waterfall or eye ailment" .)