Crossword Genius

Microscopic part for pupil, say? (8)

Ross

I believe the answer is:

eyepiece

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'pupil say?' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'microscopic part' is the wordplay.
'microscopic' becomes 'eye' (I am not sure about this - if you are sure you should believe this answer much more).
'part' becomes 'piece' (synonyms).
'eye'+'piece'='EYEPIECE'

'for' is the link.

Can you help me to learn more?

(Other definitions for eyepiece that I've seen before include "Lens(es) at the viewing end of an optical instrument" , "Given lens" , "Astronomer looks into this" , "Monocle" , "Viewing end of a microscope" .)

I've seen this clue in The Guardian.
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