Josh Portis ended an 84-yard drive by hitting Anthony McCoy with a 6-yard touchdown pass that lifted Seattle into a 17-17 tie with 4:18 to play. San Diego then fumbled the ball away on the first snap after Seattle's touchdown and Clayton scored two plays later.
The Chargers reached the Seattle 5-yard line on the ensuing drive before running out of downs.
Charlie Whitehurst was given most of the work as Seattle's quarterback and he completed 14-of-20 for 115 yards.
Philip Rivers saw limited action for the Chargers, going 5-of-6 for 87 yards and tossing a first-period touchdown to Mike Tolbert.