Old region producing gas (5)
I believe the answer is:
ozone
'gas' is the definition.
(I've seen this before)
'old region' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'region' becomes 'zone' (zone is a kind of region).
'o'+'zone'='OZONE'
'producing' is the link.
(Other definitions for ozone that I've seen before include "Bracing air of old, now seen as potentially toxic" , "Clean, bracing air" , "Sea air; toxic gas" , "Toxic form of oxygen" , "Colourless gas soluble in alkalis and cold water" .)