Agreement needed by one playing instrument (9)
I believe the answer is:
accordion
'instrument' is the definition.
(accordion is a kind of instrument)
'agreement needed by one playing' is the wordplay.
'agreement' becomes 'accord' ('accord' can be a synonym of 'agreement'**).
'needed by' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'playing' becomes 'on'.
'accord'+'i'+'on'='ACCORDION'
(Other definitions for accordion that I've seen before include "box-shaped musical instrument like melodeon" , "Bellows-operated instrument" , "Agreement starts the musical instrument" , "squeeze it while operating keys" , "Squeeze-box" .)