At sea on certain vessel (9)
I believe the answer is:
container
'vessel' is the definition.
(I know that vessel can be written as container)
'at sea on certain' is the wordplay.
'at sea' indicates anagramming the letters.
'on'+'certain'='oncertain'
'oncertain' with letters rearranged gives 'CONTAINER'.
(Other definitions for container that I've seen before include "bottle?" , "Something that holds" , "Holder" , "Metal box for carrying goods by ship" , "Sort of ship" .)