Old vessel with nitrogen? It's a gas! (5)
I believe the answer is:
argon
'it's a gas' is the definition.
(argon is a gas)
'old vessel with nitrogen?' is the wordplay.
'old vessel' becomes 'argo' (I've seen this before).
'with' means one lot of letters go next to another.
'nitrogen?' becomes 'N' (N is the chemical symbol for nitrogen).
'argo'+'n'='ARGON'
(Other definitions for argon that I've seen before include "Inert eighteen ac gas" , "Inert gas, symbol Ar" , "Gas, inert constituent of air" , "Noble gas used in light bulbs" , "noble element" .)