Fats Waller treated, removing small cataract (9)
I believe the answer is:
waterfall
'cataract' is the definition.
(synonyms)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for waterfall that I've seen before include "source of power, possibly a force in the north" , "Perpendicular descent of a river" , "Such as Niagara" , "Downpour" , "Cataract" .)