Coach Kelly Clarkson and special guest Brett Eldredge took the stage during The Voice on Tuesday night (Dec. 8) to perform their new Christmas duet, "Under the Mistletoe," providing one of the biggest highlights of the evening.Clarkson wore a long red dress, while Eldredge sported a white tuxedo jacket with black slacks and a black bowtie. They performed in front of an elaborate backdrop of extra vocalists and dancers on a set decked out with a Christmas tree, lights and oversized packages.