Etymonline says
clay
Old English clæg "stiff, sticky earth; clay," from West Germanic *klaijaz (cf. Old High German kliwa "bran," German Kleie, Old Frisian klai "clay," Old Saxon klei, Middle Dutch clei, Danish klæg "clay;" also Old English clæman, Old Norse kleima, Old High German kleiman "to cover with clay"), from PIE root *glei- "clay" (cf. Greek gloios "sticky matter;" Latin gluten "glue;" Old Church Slavonic glina "clay," glenu "slime, mucus;" Old Irish glenim "I cleave, adhere").
in Scripture, the stuff from which the body of the first man was formed; hence "human body" (especially when dead). Clay pigeon is from 1888. Feet of clay "fundamental weakness" is from Dan. ii:33.
The ideal material to fashion a form or body, it seems Cley Hill was the work of the Devil rather than the good Lord according to legend.
It may be that cley or claeg is related to
clue, the modern spelling of clew (q.v.) meaning yarn or thread used specifically to guide a traveller.