Deal with adversary, ultimately making pact (6)
I believe the answer is:
treaty
'pact' is the definition.
('treaty' can be a synonym of 'pact')
'deal with adversary ultimately' is the wordplay.
'deal with' becomes 'treat' ('treat' can be a synonym of 'deal with').
'adversary ultimately' becomes 'y' (I can't explain this - if you can you should give a lot more credence to this answer).
'treat'+'y'='TREATY'
'making' is the link.
Can you help me to learn more?
(Other definitions for treaty that I've seen before include "Formal agreement between governments" , "Ratified agreement between states" , "Pact, accord between states" , "Formal agreement between states" , "Yatter (anag.)" .)