Determined not to be given as an opponent (3,7)
I believe the answer is:
set against
'be given as an opponent' is the definition.
I can't judge whether this defines the answer.
'determined not' is the wordplay.
'determined' becomes 'set' (similar in meaning).
'not' becomes 'against' (I can't explain this - if you can you should give a lot more credence to this answer).
'set'+'against'='SET AGAINST'
'to' is the link.
Can you help me to learn more?
(Another definition for set against that I've seen is " Assail; compare, balance".)