Revelry involving Australian (10)
I believe the answer is:
saturnalia
'revelry' is the definition.
(I know that saturnalia is a type of revelry)
'involving australian' is the wordplay.
'involving' indicates an anagram.
'australian' with letters rearranged gives 'SATURNALIA'.
(Other definitions for saturnalia that I've seen before include "wild party" , "Wild Roman festival" , "orgy" , "revels" , "Old festival" .)