Line, one nailed by a fool one's played (9)
I believe the answer is:
Here is my best explanation:
'one's played' is the definition.
'line one nailed by a fool' is the subsidiary indication.
'line' becomes 'cord' (cord is a kind of line**).
'one' becomes 'i' (roman numeral).
'nailed by' is an insertion indicator.
'fool' becomes 'con'.
'cordi' inserted into 'acon' is 'accordion'.
(Other definitions for accordion that I've seen before include "Box-shaped musical instrument", "Cad I croon about a musical instrument", "Bellows-operated instrument", "Agreement starts the musical instrument", "Squeeze-box", "Free-reed aerophone", "Musical instrument", "box-shaped musical instrument like melodeon", "squeeze it while operating keys" and "It's played".)