Peace talks between Israeli and PLO negotiators are already sure to miss President Clinton's invitation to sign an agreement in Washington on Thursday. But Sam Allis reports from Jerusalem that the two sides, still struggling over what to do about 450 Jewish settlers in the West Bank city of Hebron, are now racing to beat a new deadline: Rosh Hashanah, which begins Sunday at dusk and concludes Tuesday evening. After Rosh Hoshanah come eight "days of penitence," then Yom Kippur. "If the negotiations are put on hold because of the holy days, it provides a window of opportunity for extremists to create a lot of mischief, which could really cripple the talks," Allis says. "That's why both sides are so desperate for an agreement now."