Vessel with an ensign on? (6)
I believe the answer is:
flagon
'vessel' is the definition.
(flagon is a kind of vessel)
'with an ensign on?' is the wordplay.
'with' means one lot of letters go next to another.
'an ensign' becomes 'flag' ('flag' can be a synonym of 'ensign').
'flag'+'on' is 'FLAGON'.
(Other definitions for flagon that I've seen before include "Wine jug" , "Large round bottle as for cider" , "Large wine vessel" , "Snow White companion" , "Large drinks container" .)