Denounce Conservative guarantee (7)
I believe the answer is:
censure
'denounce' is the definition.
(thesaurus)
'conservative guarantee' is the wordplay.
'conservative' becomes 'C' (UK political abbreviation).
'guarantee' becomes 'ensure' (synonyms).
'c'+'ensure'='CENSURE'
(Other definitions for censure that I've seen before include "Berate" , "Official reprimand" , "Carpet" , "Rebuke, strongly criticise" , "Formal disapproval" .)