I immediately recognized him as Abilene from The Last Picture Show.
My parents and I watched him in The Tall Man (1960-62, two seasons, 75(!) episodes). He played Billy the Kid to Barry Sullivan's Pat Garrett (the tall man in the title). I just learned that Congress pressured for the cancellation of The Tall Man because Billy (the cop killer) was portrayed as too much of a hero.
He was in several movies that I enjoyed over the years. I would say he was basically a character actor.
The photos of Clu as Billy have him right-handed. This was actually correct. Paul Newman's movie The Left Handed Gun (1958) was named for the well-known photo of the outlaw showing him standing with a pistol and a rifle. The picture had his pistol holstered to imply he was left-handed. In recent years, studying detail on the rifle, experts confirmed that the image was reversed like many pictures today. He was right-handed.