Idiot with rant about custom (9)
I believe the answer is:
tradition
'custom' is the definition.
(I know that custom can be written as tradition)
'idiot with rant about' is the wordplay.
'with' says to put letters next to each other.
'about' indicates anagramming the letters.
'idiot'+'rant'='idiotrant'
'idiotrant' with letters rearranged gives 'TRADITION'.
(Other definitions for tradition that I've seen before include "customary behaviour" , "Inherited pattern of thought or action" , "Body of beliefs and customs handed down through generations" , "Custom passed on" , "Beliefs, etc, handed down" .)