Get agreement on what will be charged for instrument (9)
I believe the answer is:
accordion
'for instrument' is the definition.
(accordion is a kind of instrument)
'get agreement on' is the wordplay.
'get' becomes 'ion' (I can't explain this - if you can you should give a lot more credence to this answer).
'agreement' becomes 'accord' (an accord is a formal agreement).
'on' is a charade indicator (letters next to each other).
'ion' put after 'accord' is 'ACCORDION'.
'what will be charged' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for accordion that I've seen before include "box-shaped musical instrument like melodeon" , "Agreement starts the musical instrument" , "Squeeze box" , "Bellows-operated instrument" , "Free-reed aerophone" .)