Conclusions of treaty saddle China with informal agreement (4)
I believe the answer is:
yeah
'informal agreement' is the definition.
(I've seen this in another clue)
'conclusions of treaty saddle china with' is the wordplay.
'conclusions of' says to take the final letters.
The last letters of 'treaty saddle china with' are 'YEAH'.
(Other definitions for yeah that I've seen before include "Casual form of yes" , "Certainly" , "The old expression of surprise" , "Affirmative response (informal)" , "Exclamation of agreement (or doubt!)" .)