Man's eyepiece (4)
I believe the answer is:
lens
This clue could be a double definition.
'man's' is the first definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'eyepiece' is the second definition.
(I've seen this in another clue)
Can you help me to learn more?
(Other definitions for lens that I've seen before include "Optical device using light to form images" , "town in France" , "Light-focusing device" , "Bender of beams" , "Part of the inner eye, or camera" .)