British vessel with unknown local supplier (7)
I believe the answer is:
brewery
'local supplier' is the definition.
(I've seen this before)
'british vessel with unknown' is the wordplay.
'british' becomes 'br' (abbreviation for British).
'vessel' becomes 'ewer' (ewer is a kind of vessel).
'with' says to put letters next to each other.
'unknown' becomes 'y' (algebra).
'br'+'ewer'+'y'='BREWERY'
(Other definitions for brewery that I've seen before include "Porter works, possibly" , "local supplier" , "Where beer is made" , "Beer producer" , "Place making beer" .)