Old Zulu one producing gas (5)
I believe the answer is:
ozone
'gas' is the definition.
(I've seen this before)
'old zulu one' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'zulu' becomes 'Z' (phonetic alphabet: alpha, bravo, charlie etc.).
'o'+'z'+'one'='OZONE'
'producing' is the link.
(Other definitions for ozone that I've seen before include "Form of oxygen in earth's upper atmosphere - there's a hole in it" , "Bracing air (colloq.)" , "Gas forming protective layer" , "Gas that screens ultraviolet radiation" , "Fresh, bracing sea air" .)