Trim fat from steak. Place steak in a shallow dish. For rub, in a small bowl combine cumin, pepper, salt, and garlic. Sprinkle rub evenly over one side of steak; rub in with your fingers. Cover and chill for 1 to 4 hours.
At least 1 hour before smoke cooking, soak wood chunks in enough water to cover. Drain before using.
In a smoker arrange preheated coals, drained wood chunks, and water pan according to the manufacturers directions. Pour water into pan. Place steak, seasoned side up, on the grill rack over water pan. Cover; smoke until steak is desired doneness. Allow about 2 hours for medium-rare (145 degrees F) or about 2-1/2 hours for medium (160 degrees degrees F). Add additional coals and water as needed to maintain temperature and moisture.
To serve, thinly slice steak across the grain. Pass Creamy Horseradish Sauce with steak. Makes 6 servings.
Creamy Horseradish Sauce
In a small bowl stir together sour cream, mustard, chives, and horseradish. In a chilled bowl beat with an electric mixer on medium speed until soft peaks form. Fold whipped cream into sour cream mixture.