A C-string one attached to instrument (9)
I believe the answer is:
accordion
'instrument' is the definition.
(accordion is a kind of instrument)
'a c-string one attached to' is the wordplay.
'string' becomes 'cord' (I've seen this before).
'one attached to' becomes 'ion' (I can't explain this - if you can you should give a lot more credence to this answer).
'a'+'c'+'cord'+'ion'='ACCORDION'
Can you help me to learn more?
(Other definitions for accordion that I've seen before include "box-shaped musical instrument like melodeon" , "Agreement starts the musical instrument" , "Free-reed aerophone" , "Cad I croon about a musical instrument" , "It's played" .)