There are many different types of predatory fish that will eat a clownfish, including sharks, eels and most predatory fish that are larger than the clownfish. However, humans are typically considered to be the clownfish's biggest predator, as humans frequently catch clownfish for aquariums and fish tanks due to their popularity.
Although many fish species will eat a clownfish if given a chance, clownfish protect themselves through a symbiotic relationship with sea anemones. The tentacles of most sea anemones sting the majority of fish if they come too close, but clownfish have developed a resistance to these stings that allows them to live in and around the tentacles with no harm. Clownfish hide within the anemones tentacles as a way to protect them from predators, but they also eat the leftovers from the fish that the anemone kills and eats.
This relationship is not just beneficial for the clownfish, as they in turn help the anemones out by keeping them clean. They do this both by eating the anemones leftovers and small pests that attempt to live within the anemone. Clownfish are also very aggressive and typically chase off other types of fish that attempt to eat the anemone's tentacles.