Jargon about old vessel (4)
I believe the answer is:
'old vessel' is the definition.
(I've seen this before)
'jargon about' is the wordplay.
'about' says the answer is hidden in the clue (the clue goes around or about the answer).
'ARGO' is hidden in the letters of 'jargon'.
(Other definitions for argo that I've seen before include "Name of ship sailed by Jason in search of the Golden Fleece" , "Old boat" , "Ancient ship" , "Old craft" , "Ship used by Jason in his search for the Golden Fleece" .)