Greek vessel briefly vanished (6)
I believe the answer is:
jargon
'greek' is the definition.
(thesaurus)
'vessel briefly vanished' is the wordplay.
'vessel' becomes 'jar' (type of container).
'briefly' means to remove the last letter.
'vanished' becomes 'gone' (gone can mean vanished or disappeared).
'gone' with its last letter removed is 'gon'.
'jar'+'gon'='JARGON'
(Other definitions for jargon that I've seen before include "Professional gibberish" , "Specialised vocabulary of a particular group" , "Cant" , "Journalese or legalese, say?" , "Bandied words, possibly" .)