So what is a clue, anyway? It was originally spelled clew and first referred to a "globular body" (thus the OED.) From there it evolved to mean a ball of thread. This was used in the myth of Theseus and the Minotaur. In this myth Theseus, that Grecian Captain America, stumbled into the confusingly twisty Labyrinth to slay the dreaded Minotaur. Theseus' admirer Adriane gave him a ball of thread--a clue--with which to find his way out of the labyrinth. As Chaucer had it (in the first recorded use of the word in 1385): "By a clewe of twyn as he hath gon The same weye he may returne a-non ffolwynge alwey the thred as he hath come." Or as we would have it, he traced the ball of twine back to where he had come.
|Theseus' modern counterpart, fighting his ancient foe.|
From there, the clew became a metaphor: the small hint that gets us out of a proverbial labyrinth. In 1605, an M. Drayton gave us this pithy one-liner: "Loosing the clew which led vs safely in, [We] Are lost within this Labyrinth of lust." Which sounds like a pretty fun Labyrinth to be lost in. The OED mentions the uses of other Labyrinths: mazes of life, of governmental departments, of obscurities. Regardless, the original metaphor of a ball of twine and Labyrinth was eventually lost and 'clue' just became a thing which helped us figure out a mystery. Its first use like this was in 1665, when a K Digby gives us this: "Seeking in the movements of the heavenly bodies for a clue to the accidents of life." A worthy place to look for a clue indeed.