Force left behind that is protecting politician (5)
I believe the answer is:
impel
'force' is the definition.
(synonyms)
'left behind that is protecting politician' is the wordplay.
'left' becomes 'l' (common abbreviation).
'behind' is a charade indicator (letters next to each other).
'that is' becomes 'i.e.' (short for Latin 'id est', 'that is').
'protecting' indicates putting letters inside (inserted letters are held or protected).
'politician' becomes 'mp' (Member of Parliament).
'ie' placed around 'mp' is 'impe'.
'l' after 'impe' is 'IMPEL'.
(Other definitions for impel that I've seen before include "Motivate" , "Urge or force to action" , "Force to act" , "Goad" , "Force into motion" .)