Eyeglass (7)
I believe the answer is:
monocle
'eyeglass' is the definition.
(I know that eyeglass can be written as monocle)
This is all the clue.
(Other definitions for monocle that I've seen before include "lens" , "One-lens seeing apparatus" , "Single eyeglass" , "Reading glass for one eye only" , "optical device" .)