Did henry clay suggest that the people of Kansas and Nebraska decide for themselves whether their territory would allow slavery

Yes, he did. Henry Clay put in place the Missouri Compromise where people in the Louisiana Territory (which included Kansas and Nebraska) were able to choose whether or not they wanted to own slaves or not, so by popular sovereignty. This was because these states were not part of the Union nor were they part of the Confederacy and some states were very divided on whether it was right to own slaves or not.