Viewer may suffer this drop (8)

I believe the answer is:

cataract

I'm a little stuck... Click here to teach me more about this clue!

'this drop' 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 "may impair the vision" , "makes it hard to see" , "might affect one's view of things" , "Waterfall and eye disease" , "that impairs vision" .)

I've seen this clue in The Telegraph.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.