Clancy refereed at the 2004 FIRA Women's European Championship and at the 2005 and 2006 Under 21 Rugby World Championships. He made his senior international debut on 30 September 2006 when he took charge of a 2007 Rugby World Cup qualifier between Uruguay and the United States. He also refereed the 2007 Churchill Cup final. In 2008 Clancy refereed his first top level internationals. On 1 June he took charge of a match between England and the Barbarians and on 21 June he refereed a match between South Africa and Italy. Clancy made his Six Nations Championship debut on 14 February 2009 when he took charge of a match between France and the Scotland. Clancy made his Rugby Championship debut on 24 July 2010 when he took charge of a match between Australia and South Africa.