A vessel? Yes, on the starboard side (3)
I believe the answer is:
jar
'a vessel?' is the definition.
(type of container)
'yes on the starboard side' is the wordplay.
'yes on the starboard' becomes 'ja' (I can't justify this - if you can you should believe this answer much more).
'side' becomes 'r' (abbreviation for right).
'ja'+'r'='JAR'
Can you help me to learn more?
(Other definitions for jar that I've seen before include "(Jam) pot" , "Glass of beer; clash" , "Grate" , "Glass of beer (colloq.)" , "drink" .)