Charge follows an agreement for an instrument (9)
I believe the answer is:
accordion
'for an instrument' is the definition.
(accordion is a kind of instrument)
'charge follows an agreement' is the wordplay.
'charge' becomes 'ion' (an ion is a charged particle).
'follows' says to put letters next to each other.
'an agreement' becomes 'accord' (accord is a kind of agreement).
'ion' after 'accord' is 'ACCORDION'.
(Other definitions for accordion that I've seen before include "Squeeze box" , "Bellows-operated instrument" , "Free-reed aerophone" , "Cad I croon about a musical instrument" , "It's played" .)