Black vessel in stream (4)
I believe the answer is:
burn
'stream' is the definition.
(a burn is a small stream)
'black vessel' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'vessel' becomes 'urn' (urn is a type of container).
'b'+'urn'='BURN'
'in' acts as a link.
(Other definitions for burn that I've seen before include "Smart" , "Char" , "Scorch - stream" , "Catch light; stream" , "Set 1 across [ALIGHT]" .)