Clay (kl?), Cassius Marcellus 1810-1903. American abolitionist and public official who was minister to Russia (1861-1862 and 1863-1869).American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.