One in the eye? (4)

I believe the answer is:

lens

'one in the eye?' is the definition.
('lens' is part of 'eye')

This is all the clue.

(Other definitions for lens that I've seen before include "optical equipment" , "something to see right through" , "Through which to view" , "Dark cloud; shining light" , "Part of photographer's equipment" .)

I've seen this clue in the Evening Standard.

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