Account, something that binds agreement (6)
I believe the answer is:
accord
'agreement' is the definition.
(an accord is a formal agreement)
'account something that binds' is the wordplay.
'account' becomes 'acc'.
'something that binds' becomes 'ord' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'acc'+'ord'='ACCORD'
Can you help me to learn more?
(Other definitions for accord that I've seen before include "Agreement - conformity" , "Match" , "Agree" , "Unanimity" , "Myra -, pianist; Rudolf -, Nazi" .)