One synonym for the phrase "clabber up" is "curdle," which refers to the process of milk or other liquids becoming thick and lumpy when exposed to heat or acid. Another similar phrase is "settle," which describes the process of solids or sediment sinking to the bottom of a liquid. "Coagulate" is another synonym, which specifically refers to the process of blood clotting or the formation of a solid mass from a liquid. Finally, "thicken" is a simple and straightforward synonym for "clabber up," describing the process of a liquid becoming more viscous or solid. All of these words describe similar processes to "clabber up" and can be used interchangeably in many contexts.