According to Entertainment Weekly, the 'My Life Would Suck Without You' singer performed an acoustic version of Tracy Chapman's 'Fast Car' with Chris Daughtry.
Daughtry later wrote on his Twitter account: "Thanks again for an amazing time last night Nashville. And BIG thanks to @kelly_clarkson and Brad Arnold for coming out!!! (sic)"
Clarkson won the first season of the Fox competition while Daughtry finished in fourth place on the fifth season, which saw Taylor Hicks crowned the winner.
Clarkson performed alongside fellow American Idol winners and finalists at the season nine finale last week, which marked the final appearance of Simon Cowell on the judging panel.