Clear Channel Communications Inc., the nation's biggest radio station owner, said it has agreed to be acquired for about $18.7 billion by an investment group. The transaction would be one of the biggest deals in which a company has been taken private, and showcases that vast sums that buyout groups have recently been able to assemble to acquire public companies. Clear Channel said the deal calls for a group led by the investment firms Thomas H. Lee Partners LP and Bain Capital Partners LLC to pay $37.60 in cash for each share of Clear Channel common stock.