Heat oven to 425°F. Place nuts in single layer on a 15 x 11 x 1-inch jelly-roll pan. Bake in 425°F oven for 6 to 7 minutes, stirring once, to lightly toast nuts. Remove from oven, set pan on wire rack.
Cover 2 large baking sheets with nonstick aluminum foil and set aside along with two dinner forks.
Combine sugar and cream of tartar in heavy medium-size saucepan. Stir in 1 cup water and cook over high heat, stirring until sugar dissolves completely and mixture starts to boil, about 4 minutes.
Boil mixture without stirring until syrup reaches 280°F on candy thermometer, about 15 minutes. Remove from heat and quickly stir in nuts. If mixture gets too thick, add up to 3 tablespoons of water and return to heat for 10 seconds.
Place prepared sheet next to saucepan. Using dinner fork, quickly scoop out nuts and scrape off with a second fork into small clusters onto prepared baking sheet. There will be syrup remaining in the pan. Let clusters cool completely, about 1-1/2 hours. Store in airtight container, at cool room temperature, for up to 3 weeks.