Old ship with nitrogen gas (5)
I believe the answer is:
argon
'gas' is the definition.
(I've seen this before)
'old ship with nitrogen' is the wordplay.
'old ship' becomes 'argo' (I've seen this in other clues).
'with' says to put letters next to each other.
'nitrogen' becomes 'n' (N is the chemical symbol for nitrogen).
'argo'+'n'='ARGON'
(Other definitions for argon that I've seen before include "Inert gas, symbol Ar" , "noble element" , "something inert and insubstantial" , "Gas, 1% of atmosphere" , "Inert gas in atmosphere" .)