Gas I located beneath Australia (5)
I believe the answer is:
ozone
'gas' is the definition.
(I know that ozone is a type of gas)
'i located beneath australia' is the wordplay.
'i' becomes 'one' (Roman numeral for one).
'located beneath' says to put letters next to each other (in a down clue, some letters go below others).
'australia' becomes 'oz' (informal name for Australia).
'one' after 'oz' is 'OZONE'.
(Other definitions for ozone that I've seen before include "Shield against radiation" , "Kind of oxygen layer in atmosphere, has hole in it" , "That precious layer" , "A gas; threatened layer in air" , "It's said to brace" .)