Instigate a crude lie about a politician (5)
I believe the answer is:
impel
'instigate' is the definition.
(thesaurus)
'a crude lie about a politician' is the wordplay.
'a crude' indicates an anagram.
'about' indicates putting letters inside.
'a politician' becomes 'MP' (Member of Parliament).
'lie' is an anagram of 'iel'.
'iel' enclosing 'mp' is 'IMPEL'.
(Other definitions for impel that I've seen before include "Urge or drive to do something" , "Send or drive forward" , "Motivate" , "Oblige" , "Spur on" .)