A C string one attached to musical instrument (9)
I believe the answer is:
accordion
'musical instrument' is the definition.
(accordion is a kind of musical instrument)
'a c string one attached' is the wordplay.
'string' becomes 'cord' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'attached' becomes 'on' (I've seen this in other clues).
'a'+'c'+'cord'+'i'+'on'='ACCORDION'
'to' acts as a link.
(Other definitions for accordion that I've seen before include "Bellows-operated instrument" , "squeeze it while operating keys" , "Cad I croon about a musical instrument" , "Agreement starts the musical instrument" , "Free-reed aerophone" .)