Waterfall, rapids (8)
I believe the answer is:
cataract
This clue is a double definition.
'waterfall' is the first definition.
(synonyms)
'rapids' is the second definition.
(thesaurus)
(Other definitions for cataract that I've seen before include "Waterfall - eye problem" , "Lens defect" , "Waterfall or eye ailment" , "Fall of water or eye ailment" , "can obstruct your view" .)