The pilot reported that while descending at about 160 kts during an instrument approach in visual meteorological conditions with the autopilot engaged, and passing through 5,000 ft msl, the airplane encountered “a sudden pocket” of severe clear air turbulence for one second, resulting in the loss of about 300 ft of altitude. A passenger in the cabin, who was in the process of restraining himself when the turbulence encounter occurred, hit his head on the cabin ceiling and sustained a serious injury. The autopilot was turned off, the pilot continued with the approach, and the airplane landed at the destination airport without further incident. The airplane did not sustain any damage from the turbulence encounter. The pilot reported there were no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation. The pilot reported that at the time of the accident, an AIRMET for moderate turbulence below 12,000 ft msl and high surface winds was active for the area. The passengers were verbally instructed by the pilot to make sure their restraint systems were fastened. The pilot activated the cabin seatbelt sign as an aural and visual warning as reinforcement.