When something has a funny taste, it means that it tastes unusual. When something is funny, it means that something is amusing and elicits laughter. A clown is a performer with a painted face that entertains others, and tries to be funny. A circus lion won't eat a clown because they taste unusual. The joke is playing on the word "funny" and its different meanings. 
A circus lion won ' t eat clowns because they taste funny .