Redundancy offensive involves only Conservative leader separately (12)
I believe the answer is:
obsolescence
'redundancy' is the definition.
'offensive involves only conservative leader separately' is the wordplay.
I cannot really see how this works, but
'only' could be 'sole' (synonyms) and 'sole' is found in the answer.
'conservative' could be 'c' (abbreviation commonly used in UK politics) and 'c' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for obsolescence that I've seen before include "going out of date" , "Process of becoming out of date" , "Out-of-date quality" .)