It prevents clear sight of waterfall (8)
I believe the answer is:
cataract
'waterfall' is the definition.
(synonyms)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for cataract that I've seen before include "one may have causing blurred vision" , "It may prevent someone seeing" , "Rush of water - eye problem" , "Waterfall and eye defect" , "Large waterfall" .)