The engagement of Kathryn Ann Nay to Dr. Lucius Dubignon Clay, III, has been announced by Dr. and Mrs. Howard Riley Nay of Tenafly, N.J., parents of the future bride. Her fiance is the son of General (USAF ret.) and Mrs. Clay, Jr. of Colorado Springs, formerly of Alexandria, Va. A September wedding is planned.

Miss Nay, an actress in New York, was graduated from the Masters School and Hollins College. Her father is an attending surgeon at St. Luke's Hospital in New York and associate clinical professor of surgery at the Columbia University College of Physicians and Surgeons.

Dr. Clay, a surgical resident at St. Luke's Roosevelt Hospital Center, was graduated from Episcopal High School, Washington and Lee University, and the University of Virginia Medical School. He served as a lieutenant with the Army Special Forces before interning at Bellevue Hospital. His father was Commander of the Seventh Air Force in Southeast Asia, Commander of the Pacific Air Forces, and served several tours in the Pentagon.

The prospective bridegroom is a grandson of Mrs. Clay of McLean, Va. and the late General Clay, Commander of the United States Forces in Europe and Military Governor of Germany following WWII; he is also grandson of the late Mr. and Mrs. Charles C. Commander of Tampa, Fla.