British member of parliament's vessel (4)
I believe the answer is:
bowl
'vessel' is the definition.
(bowl is a kind of vessel)
'british member of parliament's' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'member of parliament' becomes 'owl' ('parliament' is the collective noun for 'owl').
'b'+'owl'='BOWL'
(Other definitions for bowl that I've seen before include "Throw the ball" , "See 20" , "Basin; attack wicket" , "Wooden ball used in lawn game" , "Play with tenpins" .)