Being aboard very large boat (6)
I believe the answer is:
vessel
'boat' is the definition.
(boat is a type of vessel)
'being aboard very large' is the wordplay.
'being' becomes 'esse' (I've seen this in other clues).
'aboard' indicates putting letters inside.
'very' becomes 'v' (abbreviation).
'large' becomes 'L' (eg in clothes sizes).
'v'+'l'='vl'
'esse' put inside 'vl' is 'VESSEL'.
(Other definitions for vessel that I've seen before include "Kind of ship or a kitchen pot (6)" , "Boat or bottle, say" , "bowl?" , "Blood-carrying tube or a ship" , "Container - ship" .)