Skill overturning idiot's stupid new belief (9)
I believe the answer is:
tradition
'belief' is the definition.
(thesaurus)
'skill overturning idiot's stupid new' is the wordplay.
'skill' becomes 'art' (I've seen this before).
'overturning' says the letters should be written backwards.
'stupid' indicates an anagram (in a stupid order).
'new' becomes 'n'.
'art' in reverse letter order is 'tra'.
'idiot' with letters rearranged gives 'ditio'.
'tra'+'ditio'+'n'='TRADITION'
(Other definitions for tradition that I've seen before include "stuck in the old ways" , "Established usage" , "Practices of long standing" , "Established practice" , "Customs etc handed down" .)