Crossword Genius

Overseeing helper? (9)

Ross

I believe the answer is:

periscope

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

I cannot understand how the rest of the clue works.

(Other definitions for periscope that I've seen before include "Sub's eye?" , "tube with mirrors" , "Optical instrument" , "Submarine's viewer" , "overseer" .)

I've seen this clue in The Times.
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