Conservative failure that will come to an end (6)
I believe the answer is:
closer
'conservative failure that will come to an end' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I can't see?
'conservative failure' is the wordplay.
'conservative' becomes 'C' (UK political abbreviation).
'failure' becomes 'loser' (someone who has failed).
'c'+'loser'='CLOSER'
Can you help me to learn more?
(Other definitions for closer that I've seen before include "more reticent" , "more precise" , "One who is skilled at bringing a business transaction to a satisfactory conclusion" , "One winds up" , "within a shorter distance" .)