Theatre at York welcoming this agreement (6)
I believe the answer is:
treaty
'this agreement' is the definition.
(treaty is a kind of agreement)
'theatre at york welcoming' is the wordplay.
'welcoming' indicates the answer is hidden within the clue.
'TREATY' is hidden within 'theatre at york'.
(Other definitions for treaty that I've seen before include "Political or private agreement" , "convention?" , "A pact between countries" , "Compact; concordat" , "Written agreement between two states" .)