Australian unit making gas (5)
I believe the answer is:
ozone
'gas' is the definition.
(I've seen this before)
'australian unit' is the wordplay.
'australian' becomes 'oz' (nickname for Australia).
'unit' becomes 'one' ('one' can be a synonym of 'unit'**).
'oz'+'one'='OZONE'
'making' is the link.
(Other definitions for ozone that I've seen before include "Gas in upper atmosphere" , "Gas forming protective layer" , "Seaside air, maybe" , "Layer that protects against ultraviolet rays" , "Stratospheric layer of gas -- invigorating sea air" .)