Australian individual, a gas! (5)
I believe the answer is:
ozone
'a gas' is the definition.
(ozone is a kind of gas)
'australian individual' is the wordplay.
'australian' becomes 'Oz' (nickname for Australia).
'individual' becomes 'one' (an individual is one person).
'oz'+'one'='OZONE'
(Other definitions for ozone that I've seen before include "Condensed oxygen; sea air" , "Toxic form of oxygen" , "Atmospheric gas shielding us from ultraviolet radiation" , "Form of oxygen, O3" , "Atmospheric layer in some danger" .)