One sees what's up, in the main (9)

I believe the answer is:

periscope

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'main' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for periscope that I've seen before include "Submarine's viewer" , "Optical instrument" , "Viewing instrument in a submarine" , "Submarine viewing aid" , "Sub's eye?" .)

I've seen this clue in The Telegraph.

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