Cataract (9)
I believe the answer is:
waterfall
'cataract' is the definition.
(synonyms)
This is all the clue.
(Other definitions for waterfall that I've seen before include "One of the aqueous drops" , "source of power, possibly a force in the north" , "Downpour" , "Cataract" , "Steep descent of a river" .)