At sea, it's something superior on the side (7)

I believe the answer is:

gunwale

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'side' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe there's a link between them I don't understand?

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for gunwale that I've seen before include "Part of ship" , "Top of boat's side" , "Upper edge of ship's side" , "part of boat" , "Part of craft" .)

I've seen this clue in the Evening Standard.

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