Harmony shown by one playing instrument (9)
I believe the answer is:
accordion
'instrument' is the definition.
(accordion is a kind of instrument)
'harmony shown by one playing' is the wordplay.
'harmony' becomes 'accord' (accord is a kind of harmony).
'shown by' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'playing' becomes 'on'.
'accord'+'i'+'on'='ACCORDION'
(Other definitions for accordion that I've seen before include "squeeze it while operating keys" , "box-shaped musical instrument like melodeon" , "Agreement starts the musical instrument" , "Squeeze-box" , "Bellows-operated instrument" .)