Sink aboard vessel (6)
I believe the answer is:
flagon
'vessel' is the definition.
(flagon is a kind of vessel)
'sink aboard' is the wordplay.
'sink' becomes 'flag' (flagging is a kind of sinking**).
'aboard' becomes 'on'.
'flag'+'on'='FLAGON'
(Other definitions for flagon that I've seen before include "Large drinks container" , "Wine jug" , "Large container for serving drink" , "Large rounded bottle, may contain cider" , "Drinking vessel" .)