Clay Aiken will be anything but ''Invisible'' on the concert circuit this summer. Having just wrapped his co-headlining tour with fellow ''American Idol'' star Kelly Clarkson, the red-haired crooner will go it alone on a 28-date summer tour that, true to his down-home roots, includes eight dates at state and county fairs. According to his website, Aiken's tour will begin July 8 in Grand Forks, N.D. and keep him on the road through Sept. 19 (Springfield, Mass.), with one final date a month later in Raleigh, N.C. He also has a pre-tour slot at radio station KIIS' ''Wango Tango'' show in Pasadena, Calif., on May 15, which will see him sharing a bill with such diverse acts as Janet Jackson, Jessica Simpson, JC Chasez, Maroon 5, Big Boi, Hilary Duff, J-Kwon, and ''Idol'' reject William Hung. And to think we all laughed at the notion of Hung taking his place beside the other ''Idol''-minted stars.