SOMEONE SLIPPED poison into Clara von Hohenkammer`s herb tea. All the possible suspects, each with a credible motive for homicide, were in the dining room of the rich, aging German actress`s Rosedale mansion when the fatal sip sent her crashing to the floor, and all must endure the dogged interrogation of Inspector John Sanders of the Toronto police, who is abetted by his somewhat disaffected lady friend, Harriet Jeffries. Woven into Murder in a Good Cause (Viking, 247 pages, $22.95 cloth), Medora Sales third whodunit, is an obligatory sub-plot involving the fencing of precious art works; the savvy reader knows that both crimes will somehow be shown to be connected. Clearly Sale has down cold the mystery idiom that Agatha Christie perfected and others have duly imitated, but in her latest book she adds little to the genre. In prose as flat and familiar as the back of your hand, Sale spins out the complexities of the plot competently enough, and all her characters come with detailed dossiers; %,that they lack, however, are individual voices of their own. The faltering love affair between the two sleuths seems particularly perfunctory padding here, when clearly the only reason one pursues Sale`s story is to find out who did in Clara von Hohenkammer. Even the most punctilious reader may peek ahead to the last chapter.