Deal with unknown character, making agreement (6)
I believe the answer is:
treaty
'agreement' is the definition.
(I know that treaty is a type of agreement)
'deal with unknown character' is the wordplay.
'deal with' becomes 'treat' (synonyms).
'unknown character' becomes 'y' (algebra).
'treat'+'y'='TREATY'
'making' acts as a link.
(Other definitions for treaty that I've seen before include "Formal agreement between states" , "International agreement" , "settlement" , "alliance" , "A pact between countries" .)