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