Black gear for two-masted vessel (4)
I believe the answer is:
brig
'two-masted vessel' is the definition.
(I've seen this in another clue)
'black gear' is the wordplay.
'black' becomes 'b' (abbreviation as in pencil classifications).
'gear' becomes 'rig' (rig is a kind of gear).
'b'+'rig'='BRIG'
'for' is the link.
(Other definitions for brig that I've seen before include "Two-masted square-rigged vessel" , "'Two-masted,square-rigged ship (4)'" , "Two-master; army officer (abbr.)" , "Two-masted square-rigger" , "Sailing vessel or lock-up on board" .)