Problem with vision producing fall (8)

I believe the answer is:

cataract

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'fall' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for cataract that I've seen before include "It may prevent someone seeing" , "Waterfall - eye problem" , "A real downpour" , "Cascade; eye defect" , "Falls" .)

I've seen this clue in The Guardian.

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