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'.