Vessel with second-rate gear (4)
I believe the answer is:
'vessel' is the definition.
(brig is a kind of vessel)
'with second-rate gear' is the wordplay.
'with' means one lot of letters go next to another.
'second rate' becomes 'b'.
'gear' becomes 'rig' (rig is a kind of gear).
'b' put next to 'rig' is 'BRIG'.
(Other definitions for brig that I've seen before include "Ship; ship's prison" , "Two-masted square rigger" , "Two-master; army officer (abbr.)" , "Two-masted square-rigger" , "'Two-masted,square-rigged ship (4)'" .)