Crossword Genius

Replacement viewer switched to monocle (8)

Ross

I believe the answer is:

eyeglass

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

'to monocle' is the definition.
(synonyms)

'replacement viewer switched' is the wordplay.
I cannot really see how this works, but
'viewer' could be 'eye' (organ which sees or views) and 'eye' is found within the answer.
The remaining letters 'glass' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

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

I've seen this clue in The Sun.
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