Clark Terrell was a Starfleet captain who lived in the 23rd century.
In 2285, Captain Terrell was commanding the starship Reliant and assigned to support the Genesis Project by searching for a suitable planet on which to test the Genesis Device. He had never met Admiral James T. Kirk, but knew of him through his first officer, Pavel Chekov.
During a surface survey of what they believed to be Ceti Alpha VI, Terrell and Chekov were captured by Khan Noonien Singh and his followers, who had been exiled there some 15 years previously. (TOS: "Space Seed") Terrell and Chekov discovered that they were in fact on Ceti Alpha V, and that Ceti Alpha VI had actually exploded only six months after Khan was marooned.
In order to force Terrell to reveal their purpose on the surface, Khan infested him and Chekov with parasitic Ceti eels, which affected the cerebral cortex and allowed Khan to control them by suggestion. Using Terrell, Khan took control of the Reliant, stranded the Starfleet crew on the surface, and attacked spacelab Regula I, the location of the research for the Genesis Project. When Khan failed to find Genesis, he abandoned Terrell and Chekov on the station as a trap for Admiral Kirk.
The pair were rescued a short time later by a boarding party from the USS Enterprise, and subsequently discovered where the Genesis materials had been hidden – underground on Regula I itself. With the Genesis materials uncovered, Terrell contacted Khan with the coordinates, allowing Khan to steal the powerful Genesis torpedo.
Terrell committing suicide David Marcus rushed Terrell in a fit of anger, who fired his phaser, and Saavik leaped onto David to push him out of the way. The blast instead hit Jedda, who was vaporized. Khan then ordered Terrell to kill Admiral Kirk. However, Terrell resisted this last deadly order. Under immense pain from the Ceti eel, Terrell turned his phaser on himself, choosing to commit suicide rather than murder another Starfleet officer.