Agreement calls for celebration by party at last (6)
I believe the answer is:
treaty
'agreement' is the definition.
(treaty is a kind of agreement)
'celebration by party at last' is the wordplay.
'celebration' becomes 'treat' (I've seen this before).
'by party at last' becomes 'y' (I can't justify this - if you can you should give a lot more credence to this answer).
'treat'+'y'='TREATY'
'calls for' acts as a link.
Can you help me to learn more?
(Other definitions for treaty that I've seen before include "alliance" , "Yatter (anag.)" , "Compact; concordat" , "Formal agreement between two or more countries" , "International agreement" .)