Metal rim around an eyepiece (5)
I believe the answer is:
pupil
'eyepiece' is the definition.
(pupil is a piece or part of the eye)
'metal rim around' is the wordplay.
'metal' becomes 'pu' (I can't justify this - if you can you should believe this answer much more).
'rim' becomes 'lip' (both can mean the edge of something).
'around' says the letters should be written in reverse.
'lip' back-to-front is 'pil'.
'pu'+'pil'='PUPIL'
'an' acts as a link.
Can you help me to learn more?
(Other definitions for pupil that I've seen before include "one may see through!" , "Learner in the eye?" , "One studying" , "Scholar" , "part of my anatomy" .)