One manufacturing nitrogen gas (4)
I believe the answer is:
neon
'gas' is the definition.
(neon is a gas)
'one manufacturing nitrogen' is the wordplay.
'manufacturing' indicates an anagram.
'nitrogen' becomes 'N' (N is the chemical symbol for nitrogen).
'one' is an anagram of 'neo'.
'neo'+'n'='NEON'
(Other definitions for neon that I've seen before include "None (anag.)" , "illuminating agent" , "Could it brighten proceedings" , "Inert gas, symbol Ne" , "Sign gas" .)