Force terrible lie about politician (5)
I believe the answer is:
impel
'force' is the definition.
('impel' can be a synonym of 'force')
'terrible lie about politician' is the wordplay.
'terrible' indicates anagramming the letters.
'about' is an insertion indicator.
'politician' becomes 'mp'.
'lie' is an anagram of 'iel'.
'iel' enclosing 'mp' is 'IMPEL'.
(Other definitions for impel that I've seen before include "Send or drive forward" , "Motivate" , "Oblige" , "Drive or coerce" , "Urge forward, drive" .)