Jail for a sailor (4)
I believe the answer is:
brig
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">jail for a sailor</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)