It is the first major golf tournament of the year on the European Tour.
Clarke last played three weeks ago at the Houston Open. He withdrew to be with his ailing wife, Heather, who has battled cancer for several years.
New Zealand's Michael Campbell, Paul Casey of England and Jarmo Sandelin of Sweden are tied for second place at 5-under 67. In a tie for third place at 4-under-par 68 heading into the second round are David Higgins of Ireland, Scotland's Andrew Coltart, Johan Edfors of Sweden, and Ian Poulter, Oliver Watson and Gary Emerson, all from England.