Iron is a reference to the Roman empire, which was split into 10 nations, once the western Roman Empire did fall. Those 10 nations are described as part clay, as they do not have the strength they had under Rome, as they be divided against each other, and will not be able to unite, as they will break off from each other, just like clay breaks easy. That is the interpretation from Daniel. These part clay kingdoms are going to last until Jesus second coming, as he is the one who is going to put an end to their rule.