Black vessel set alight (4)
I believe the answer is:
burn
'set alight' is the definition.
(I've seen this before)
'black vessel' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'vessel' becomes 'urn' (urn is a type of container).
'b'+'urn'='BURN'
(Other definitions for burn that I've seen before include "northern stream" , "Combust" , "Severely overcook" , "Scottish stream" , "Destroy by fire" .)