Old Greek vessel with first of natural gas (5)
I believe the answer is:
argon
'gas' is the definition.
(argon is a gas)
'old greek vessel with first of natural' is the wordplay.
'old greek vessel' becomes 'argo' (I can't justify this - if you can you should give a lot more credence to this answer).
'with' says to put letters next to each other.
'first of natural' becomes 'n' (ast letter of 'natural').
'argo'+'n'='ARGON'
Can you help me to learn more?
(Other definitions for argon that I've seen before include "elementary" , "Gas, inert constituent of air" , "Inert gas, symbol Ar" , "Inert gas in atmosphere" , "something inert and insubstantial" .)