Agreement to have nothing charged for instrument (6)
I believe the answer is:
accord
'agreement' is the definition.
(accord is a kind of agreement)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for accord that I've seen before include "Correspond" , "Be in harmony or consistent" , "Unanimity" , "Official agreement; harmony" , "binding agreement" .)