Party is a natural disaster (10)
I believe the answer is:
saturnalia
'party' is the definition.
(thesaurus)
'is a natural disaster' is the wordplay.
'disaster' is an anagram indicator.
'is'+'a'+'natural'='isanatural'
'isanatural' is an anagram of 'SATURNALIA'.
(Other definitions for saturnalia that I've seen before include "orgy" , "Old festival" , "Ancient Roman December festival" , "wild party" , "Wild Roman festival" .)