According to the upcoming issue of National Journal, that man in cowboy hat and yellow-striped riding trousers seen in Rock Creek Park riding the magnificent white stallion Amadeus, a gift to the president from the Austrian government, was Interior Secretary William P. Clark . . . Sometime after the elections, Hendrik Hertzberg has said he wants to relinquish the editorship of The New Republic. The word around town is that Michael E. Kinsley, who writes the TRB column and was once editor, will take on that post again . . . Jihan Sadat, widow of the late Egyptian president Anwar Sadat, will be a distinguished professor in residence at the American University next spring. She will conduct a symposium, "Women in the Changing World," with former first lady Betty Ford as her first guest speaker . . . Much of Washington's art and social establishment turned out for artist Marjorie Phillips 90th birthday celebration last night. Among those toasting her were Oatsie Charles, Caroline Simmons, Mary Fritchey, Amie and Huntington Block, and Franz and Virginia Bader as well as family members . . . It's probably as scientific as most political polls. Patrons at Laurel Race Course will be asked next week to select Republican elephant or Democratic donkey cups when they order drinks at the track. The theory is the horse players will reflect who will win Nov. 6. Horse players, however, are famous for staring gloomily at losing betting tickets in their hands and saying they should have bet on the other horse . . .