The Clear Lake trail is an up-and-back run. No loops here. You go up to Clear Lake and you head back down. The turnoff to the lake is south of Ouray about 7 miles from the top of Red Mountain Pass. You'll turn right onto South Mineral Access Road heading west and follow this road about four miles until you reach a sign that indicates the way to Clear Lake. You'll head right and travel another four miles before you arrive at Clear Lake.
The reason for the rating is primarily the steep set of switchbacks on narrow shelf road towards the end. While not as steep or hairy as those on Yankee Boy trail, these require some caution and it may be best for the first timer to ride with someone to the top as there is a pulloff at the start of this final climb where a few FSJs could be parked temporarily.
Sources
The 4X4 Trail Book-Number 1, Colorado Association of 4 Wheel Drive Clubs, Inc
Mountain view near Clear Lake trail
Clear Lake (left), old cabin near the trail (right)