Mimic arrives on board vessel (6)
I believe the answer is:
parrot
'mimic' is the definition.
(thesaurus)
'arrives on board vessel' is the wordplay.
'arrives' becomes 'arr' (I've seen this in other clues).
'on board' is an insertion indicator.
'vessel' becomes 'pot' (pot is a kind of vessel**).
'arr' going within 'pot' is 'PARROT'.
(Other definitions for parrot that I've seen before include "Tropical bird, may talk" , "18's [BIRD]" , "A flighty creature" , "Mimic speech like colourful bird" , "Sick as a -" .)