Clif was the name of a Trill symbiont. Across its various hosts, the symbiont served in the Trill Private Service and later Starfleet (following Trill's admission into the Federartion) since the late 22nd century.
The last host of the symbiont was Captain Gohod, who perished with the rest of his crew at the hands of the intellivore. (novel: Star Trek: The Next Generation: "Intellivore")