Edwin is in Fayette Co Indiana in 1850 Census, in Boone, Iowa in 1856 Iowa State Census, 1860 finds him in Lexington Mo as you pointed out. 1870 has him in Grape Grove, Ray, MO. He was born 1825 in Virginia to James Clark. First wife was Maria L who died between 1870 and 1873 when he married Sarah Jane Huffman and had 2 children by her before his death in 1882. 1880 Census has him married to Sarah J and living in Carroll Co MO

There is a Co E. 13th Iowa Infantry (Union) entry for an Edwin Clark. no further info NPS Soldiers and Sailors listing.
There is a Co A. 5th Reg MO Infantry Vol (Missouri State Guard 3rd Division) see:
http://www.sos.mo.gov/TIF2PDFConsumer/DispPDF.aspx?fTiff=/archives/AdjutantGeneral/Civil_War/ServiceCards/s907/1434.tif&Fln=S76825.pdf

FWIW
John R.