Jail for a sailor (4)
I believe the answer is:
brig
'jail for a sailor' 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 "Sailing vessel or lock-up on board" , "Crew can" , "Two-masted square-rigged sailing ship" , "Two-masted ship; gaol" , "Sailing-vessel" .)