 Clackson attended Peters Township High School in McMurray, Pennsylvania, and played for the school's varsity ice hockey team, winning two Pennsylvania State AA Ice Hockey Championships.He was a teammate of NHLer Christian Hanson at Peters Township.After high school, he played two seasons of Junior A with the Chicago Steel of the United States Hockey League (USHL) and was selected 215th overall in the 2005 NHL Entry Draft by the Philadelphia Flyers.