Winnie,
From an inquiry by the Ok. board of pensions commision dated Sep 14, 1915.
No records where found for Isham J H Clark in Wood's 24th Regt. Miss. Cav. aka 1st Regt. Miss. Cav. There was a Isham J. H. Clark in Co.E, 24th Regt. Miss. Infantry, enlisted Sep 15, 1861, age 22. On roll of this Co. dated Feb 29, 1864, he is reported as Absent-captured at Lookout Mt. Nov 24,1863. Union POW records show a J. J. Clark also listed as Isaac J. Clark (these names not found on roster) of 24th Regt. Miss Inf. as Captured at Chattanooga, Tn. Nov 24, 1863. Exchanged May 25, 1865 at New Orleans, La.
I found nothing that would suggest he was a Union soldier.
Mike