Replacement viewer switched to monocle (8)
I believe the answer is:
eyeglass
'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" .)