The deck is going up and down (4)
I believe the answer is:
poop
This clue is a double definition.
'the deck' is the first definition.
(poop deck is a kind of deck)
'going up and down' is the second definition.
'is' acts as a link.
(Other definitions for poop that I've seen before include "Exhaust" , "Fool" , "Tony" , "Deck at the rear of a ship" , "Whichever way you see them, the facts" .)