I found this information at Ancestry.com doing a search for Willis McClanahan:

William "Willis" McClanahan
Poole Williams family tree dec 06 25000
Birth: 17 Jul 1800 - Virginia
Marriage: 27 May 1823 - Virginia
Death: 26 Jan 1864 - Smith County (Smith), Tennessee
Parents: Thomas Marshall McClanahan, Nancy Ann Green
Spouse: Margaret "Peggy" H. Rollins

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Thomas McClanahan, son of Rev William and Mary Marshall McClanahan, from Westmoreland Co., VA to Greenville, SC.
There is a Larry D. McClanahan in Nashville, TN, who is very knowledgeable about Thomas, who died 1845 in Franklin, KY. I've seen his grave in Greenlaw? Cemetery. He had 22 children by 2 wives, one of whom was Nancy Green.

However, I think your best research might be directed toward Philip J and Althia McClanahan.

Did you mean Samuel McClanahan of Jackson, TN? He came from Lauren, SC to west TN about 1830. His brother, John R. McClanahan, ran the Memphis newspaper during the Civil War. What is your family's thinking that you might be related to them? I am curious to find out how! I've thought we were too!
John McClanahan