Agreement to one playing instrument (9)
I believe the answer is:
accordion
'instrument' is the definition.
(accordion is a kind of instrument)
'agreement to one playing' is the wordplay.
'agreement' becomes 'accord' ('accord' can be a synonym of 'agreement'**).
'to one' becomes 'i' (Roman numeral).
'playing' becomes 'on'.
'accord'+'i'+'on'='ACCORDION'
(Other definitions for accordion that I've seen before include "Free-reed aerophone" , "It's played" , "squeeze it while operating keys" , "Squeeze-box" , "box-shaped musical instrument like melodeon" .)