Clifford Jackson was an outstanding right-handed pitcher and left-handed hitter for Oil Hill, Midian and Skelly from 1921 through the mid-1930s. Born in Pine Bluff, Arkansas in 1900, his family moved to Olathe in 1911. He was pitching for an Olathe team in 1921 when he received an offer to pitch for Oil Hill and moved to El Dorado. He later received an offer to pitch for the Pittsburgh Pirates, but turned it down because he was making more money pitching for Oil Hill and Skelly.

In the 1920s, Oil Hill and the Skelly Oilers played in the Kansas Independent League. Other teams in the league were the Wichita Advertisers, Newton Railroaders, Winfield Roxannas and Augusta Oilers. Skelly also took on a number of top non-league opponents from the area and usually won. Those opponents included Hutchinson and Ponca City, the Oklahoma Oil Belt League champions. On a number of occasions for both Oil Hill and Skelly, Clifford would pitch both games of a Sunday doubleheader.

Clifford was known for a wicked curve and also threw a spit ball long before Gaylord Perry was born. Losses were rare for Jackson in his mound career. One season while pitching for Oil Hill, he beat the powerful Skelly team in 7 out of 8 starts. A career highlight came in one of his rare losses when Skelly hosted the Kansas City Monarchs at El Dorado's Wonderland Park on August 15, 1926. Wonderland Park was located roughly two blocks straight north of the current stadium and a crowd of at least 1,400 packed the park for the contest. Jackson scattered 11 hits over 9 innings and struck out 8, but Skelly lost 8-1. The big inning was the second when the Monarchs got 3 singles and a triple to go with El Dorado's only 2 errors of the game to score 5 runs. Another name that will be familiar to El Dorado natives who also played for Skelly that day was their 18 year old left fielder who had just graduated from El Dorado High School that spring, Bob McCollum.

Late in his career with the Skelly Oilers, Clifford became a mentor to another young pitcher who would also be inducted in the El Dorado Baseball Hall of Fame, Hubert Orndorff. Hubert, in turn, mentored his sons, Vernon and Dick who are also members of the El Dorado Baseball Hall of Fame.

Clifford married Minnie Kerr in Olathe in 1917 and enlisted in the United States Army shortly after. With the Army, he served on the U.S.-Mexico border. Clifford and his wife had 13 children and Clifford continued to live in El Dorado until his death in 1981.