A clue was first a ball of thread. The old word was clew, and the one that changed its meaning was the clew Ariadne gave Theseus, which he unwound behind him going into the Labyrinth so he could follow it back out; by the 1620s the word meant anything that points the way. What I like is that the original clue didn’t solve anything. It never told him where the Minotaur was, only where he had already been.