Naval hoosegow (4)
I believe the answer is:
'naval hoosegow' 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 "Two-masted square rigger" , "Ship; ship's prison" , "'Two-masted,square-rigged ship (4)'" , "Sailing vessel or lock-up on board" , "Sailing-vessel" .)