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 wordplay.
'line' becomes 'cord' (cord is a kind of line).
'one' becomes 'i' (Roman numeral).
'nailed by' indicates putting letters inside.
'fool' becomes 'con'.
'cordi' inserted within '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".)