The captain of a Boeing 737-700 reported that prior to starting a descent, he received reports of moderate turbulence in the area of the arrival airport. He said that he instructed the cabin attendants to secure the cabin early in anticipation of turbulence during the approach. The injured flight attendant related that before the flight crew initiated the descent, the attendants were asked to ready the cabin early in anticipation of turbulence. She said that as she walked to the back of the airplane, and just before she was to be seated, she noticed that a few remaining service carts located in the aft galley were not locked down. She stated that as she locked down one of the last remaining service carts, ‘the floor just fell out from under my feet, my head hit the ceiling, and I landed back on the floor, on my right leg.’ The flight attendant sustained a broken right ankle. Probable Cause: An in-flight encounter with clear air turbulence.