Pirate’s associate arriving aboard vessel (6)
I believe the answer is:
parrot
'pirate's associate' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'arriving aboard vessel' is the wordplay.
'arriving' becomes 'arr' (abbreviation used in transport).
'aboard' means one lot of letters goes inside another.
'vessel' becomes 'pot' (type of container).
'arr' put into 'pot' is 'PARROT'.
Can you help me to learn more?
(Other definitions for parrot that I've seen before include "Ape" , "Tropical bird noted for mimicking speech" , "Brightly coloured tropical bird" , "Imitate like tropical bird" , "Mechanically repeat; bird" .)