Motivate to lie terribly about a politician (5)
I believe the answer is:
impel
'motivate' is the definition.
(I've seen this in another clue)
'lie terribly about a politician' is the wordplay.
'terribly' indicates an anagram.
'about' means one lot of letters goes inside another.
'a politician' becomes 'MP' (Member of Parliament).
'lie' with letters rearranged gives 'iel'.
'iel' placed around 'mp' is 'IMPEL'.
'to' acts as a link.
(Other definitions for impel that I've seen before include "Make" , "Urge or force to action" , "Send or drive forward" , "Oblige" , "'Drive, force (5)'" .)