Gas in mythological vessel, nitrogen (5)
I believe the answer is:
argon
'gas' is the definition.
(argon is a gas)
'mythological vessel nitrogen' is the wordplay.
'mythological vessel' becomes 'Argo' (ship in Greek myth).
'nitrogen' becomes 'N' (N is the chemical symbol for nitrogen).
'argo'+'n'='ARGON'
'in' acts as a link.
(Other definitions for argon that I've seen before include "Gas used in electric lights" , "rare gas" , "something inert and insubstantial" , "Inert eighteen ac gas" , "noble element" .)