Crossword Genius

Lens used to observe ocular prosthesis having exchanged components (8)

Ross

I believe the answer is:

eyeglass

'lens used to observe' is the definition.
(telescope-like device)

I don't understand the rest of the clue.

(Other definitions for eyeglass that I've seen before include "Monocle, or spectacles in plural" , "Aid to vision" , "Lens for aiding defective vision" .)

I've seen this clue in The Independent.
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