Ignite British vessel (4)
I believe the answer is:
burn
'ignite' is the definition.
(thesaurus)
'british vessel' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'vessel' becomes 'urn' (urn is a kind of vessel).
'b'+'urn'='BURN'
(Other definitions for burn that I've seen before include "Go up in flames; stream" , "watercourse" , "runner?" , "Small stream in Scotland" , "northern stream" .)