Crossword Genius

With which one sees stars become smaller (9)

Ross

I believe the answer is:

telescope

I'm a little stuck... Click here to teach me more about this clue!

'become smaller' is the definition.
Although both the answer and definition are verbs in their base form, I can't understand how one could define the other.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for telescope that I've seen before include "Astronomer's instrument" , "Instrument for viewing from afar" , "Instrument to magnify the image of distant objects" , "Compress like optical instrument that magnifies" , "Magnifier of distant objects" .)

I've seen this clue in The Independent.
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