Some of the greatest jazz stars shared the spotlight with the nation’s best-known amateur saxophonist in a late-night celebration of “America’s classical music” on the White House lawn.At the end of a two-hour concert Friday, a tieless President Clinton grabbed his sax and joined Herbie Hancock, Grover Washington Jr., Wynton Marsalis, Joe Williams and a dozen others in a jazz and blues jam. Clinton, drenched with sweat on the muggy evening, waded into the crowd of 500 invited guests to shake hands and sign autographs. “Jazz is really America’s classical music,” he said. “Like our country itself and especially like the people who created it, jazz is a music born of struggle but played in celebration.” The concert was staged to celebrate the 40th anniversary of the Newport Jazz Festival. PBS will broadcast some of the performances Sept. 12. The evening also marked the return of “In Performance at the White House,” a public television series begun in the Carter era but halted during the Bush administration.