Conservative offer is one in opposition (9)
I believe the answer is:
contender
'one in opposition' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'conservative offer' is the wordplay.
'conservative' becomes 'con' (abbreviation for Conservative).
'offer' becomes 'tender' (tender can mean to offer something).
'con'+'tender'='CONTENDER'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for contender that I've seen before include "One seeking title" , "Candidate" , "One struggling" , "Trend once (anag.)" , "One striving" .)