Agreement to deal with enemy ultimately (6)
I believe the answer is:
treaty
'agreement' is the definition.
(I know that agreement can be written as treaty)
'deal with enemy ultimately' is the wordplay.
'deal with' becomes 'treat' (synonyms).
'enemy ultimately' becomes 'y' (I can't justify this - if you can you should give a lot more credence to this answer).
'treat'+'y'='TREATY'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for treaty that I've seen before include "Negotiation" , "'Pact, accord (6)'" , "Formal agreement between two or more countries" , "Covenant" , "Written agreement between two states" .)