Black bird in vessel (4)
I believe the answer is:
bowl
'vessel' is the definition.
(bowl is a kind of vessel)
'black bird' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'bird' becomes 'owl' (owl is a kind of bird).
'b'+'owl'='BOWL'
'in' acts as a link.
(Other definitions for bowl that I've seen before include "US stadium for sporting or musical events" , "Greatly surprise" , "Throw the ball" , "See 20" , "Round deep dish" .)