Deal with party, ultimately making pact (6)
I believe the answer is:
treaty
'pact' is the definition.
(synonyms)
'deal with party ultimately' is the wordplay.
'deal with' becomes 'treat' (synonyms).
'party 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' acts as a link.
(Other definitions for treaty that I've seen before include "International agreement" , "Political or private agreement" , "settlement" , "Written agreement between two states" , "'Pact, accord (6)'" .)