Gas from Australia, unique (5)
I believe the answer is:
ozone
'gas' is the definition.
(I know that ozone is a type of gas)
'australia unique' is the wordplay.
'australia' becomes 'oz' (informal name for Australia).
'unique' becomes 'one' (synonyms).
'oz'+'one'='OZONE'
'from' acts as a link.
(Other definitions for ozone that I've seen before include "Gas forming screen against ultraviolet radiation in the stratosphere" , "Gas in upper atmosphere" , "Bracing air of old, now seen as potentially toxic" , "A poisonous gas; seaside air" , "bracing atmosphere" .)