Crossword Genius

Old region producing gas (5)

Ross

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" .)

I've seen this clue in The Telegraph.
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