Spoil broadcast by Independent politician (6)
I believe the answer is:
impair
'spoil' is the definition.
(synonyms)
'broadcast by independent politician' is the wordplay.
'broadcast' becomes 'air' (synonyms).
'by' is a charade indicator (letters next to each other).
'independent' becomes 'i'.
'politician' becomes 'mp' (Member of Parliament).
'i'+'mp'='imp'
'air' put after 'imp' is 'IMPAIR'.
(Other definitions for impair that I've seen before include "Harm" , "Spoil, degrade" , "Hurt or handicap" , "Hinder" , "Reduce" .)