Ship's jail (4)
I believe the answer is:
brig
'ship's jail' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for brig that I've seen before include "A ship; army officer (abbr.)" , "staff officer briefly used" , "Ship; a ship's jail" , "Two-masted square-rigged vessel" , "Sailing-vessel" .)