Sailor’s confinement (4)
I believe the answer is:
brig
'sailor's confinement' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for brig that I've seen before include "Crew can" , "officer contracted" , "Two-masted square-rigged sailing ship" , "Sailing vessel or lock-up on board" , "A ship; army officer (abbr.)" .)