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