Structure on sailing ship going up and down (4)
I believe the answer is:
poop
This clue is a double definition.
'structure on sailing ship' is the first definition.
(poop deck)
'going up and down' is the second definition.
(Other definitions for poop that I've seen before include "Deck at stern of ship" , "Exhaust" , "High deck at old sailing ship's stern" , "Palindromic part of a ship" , "Whichever way you see them, the facts" .)