Crossword Genius

Producing double vision? (7)

Ross

I believe the answer is:

bifocal

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'double vision?' is the definition.
(I've seen this in another clue)

I don't understand the rest of the clue.

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(Other definitions for bifocal that I've seen before include "Like two-part lenses" , "that's seen through" , "Lens with two prescriptions" , "Allowing for distant and near vision" , "Type of spectacles for long and short sight" .)

I've seen this clue in The Irish Independent.
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