Deal with your first agreement (6)
I believe the answer is:
treaty
'agreement' is the definition.
(I know that treaty is a type of agreement)
'deal with your first' is the wordplay.
'deal with' becomes 'treat' (synonyms).
'your first' becomes 'y' (1st letter of 'your').
'treat'+'y'='TREATY'
(Other definitions for treaty that I've seen before include "International agreement" , "Contract, agreement" , "settlement" , "Formal agreement between nations" , "A pact between countries" .)