Submarine's viewer (9)

I believe the answer is:

periscope

'submarine's viewer' is the definition.
(I've seen this before)

This is the entire clue.

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

I've seen this clue in The Star Tribune and The Times.

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