Agree with one playing instrument (9)
I believe the answer is:
accordion
'instrument' is the definition.
(accordion is a kind of instrument)
'agree with one playing' is the wordplay.
'agree with' becomes 'accord' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'playing' becomes 'on'.
'accord'+'i'+'on'='ACCORDION'
(Other definitions for accordion that I've seen before include "It's played" , "Bellows-operated instrument" , "Cad I croon about a musical instrument" , "squeeze it while operating keys" , "Squeeze-box" .)