British vessel in stream (4)
I believe the answer is:
burn
'stream' is the definition.
(a burn is a small stream)
'british vessel' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in BBC).
'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 "Catch light; stream" , "Combust" , "Scottish stream" , "runner?" , "Scottish brook" .)