Agreement on including one instrument (9)
I believe the answer is:
accordion
'instrument' is the definition.
(accordion is a kind of instrument)
'agreement on including one' is the wordplay.
'agreement' becomes 'accord' ('accord' can be a synonym of 'agreement'**).
'including' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'accord'+'on'='accordon'
'accordon' enclosing 'i' is 'ACCORDION'.
(Other definitions for accordion that I've seen before include "Squeeze-box" , "Cad I croon about a musical instrument" , "Agreement starts the musical instrument" , "It's played" , "Squeeze box" .)