Mrs. Clark Clifford is an experienced Washington hostess with a happy tendency to collect guests' and her own family recipes. Not surprisingly there are plenty of dishes to the buffet born -- casseroles, aspics, salads and striking desserts. Beginning with canapes and quiches, Mrs. Clifford offers an enormous number of recipes -- some ""foreign,"" some haute cuisine but all manageable for the competent cook, and all respectable guest fare. Occasionally credits are appended -- from predictables like Mrs. Muskie's Maine Clam Chowder and Mrs. Nixon's Pumpkin Pie to that busy housewife Mrs. Onassis' Fish Chowder. Not a word as yet from South Dakota.