In the next verse : ثُمَّ جَعَلْنَاهُ نُطْفَةً (then We made him a sperm-drop) this same point has been mentioned differently. The meaning of all this is that the first life i.e. Sayyidna 'Adam علیہ السلامٍ was created from earth and continued further with the sperm which is a refined element of the same earth. This is how a majority of commentators have explained this verse. Another view is that (extract of clay) may also mean human seed because it is produced from the food which a man takes and the food grows out of the earth.