Idiot with rant about historical convention (9)
I believe the answer is:
tradition
'historical convention' is the definition.
'tradition' can be an answer for 'convention' (thesaurus). I'm not sure about the 'historical' bit.
'idiot with rant about' is the wordplay.
'with' says to put letters next to each other.
'about' indicates an anagram.
'idiot' after 'rant' is 'rantidiot'.
'rantidiot' with letters rearranged gives 'TRADITION'.
Can you help me to learn more?
(Other definitions for tradition that I've seen before include "Inherited pattern of thought or action" , "Transmitted customs" , "Custom, long-standing practice" , "Body of beliefs and customs handed down through generations" , "Beliefs, etc, handed down" .)