Keep quiet aboard ship (4,4)
I believe the answer is:
pipe down
'keep quiet' is the definition.
(to pipe down is to be quiet)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for pipe down that I've seen before include "Stop talking" , "Sh!" , "Say no more!" , "Be quiet!" , "make less noise" .)