Keep quiet aboard ship (4,4)

I believe the answer is:

pipe down

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'keep quiet' is the definition.
(to pipe down is to be quiet)

I cannot understand how the rest of the clue works.

(Other definitions for pipe down that I've seen before include "Stop talking" , "Sh!" , "Say no more!" , "Be quiet!" , "make less noise" .)

I've seen this clue in The Guardian, The Irish News and The Globe and Mail.

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