something that helps to solve a problem or mystery or to indicate the perpetrator of or motive for a crime
a cryptic phrase, anagram, etc that has to be solved as part of a crossword puzzle
not have a clue
to know nothing; to have no idea
I haven't a clue where he's gone
to be incompetent
Middle English variant of clew1, from Old English cliewen rounded mass, ball of thread. The forms clew and clue were used more or less interchangeably until at least the 18th cent. Sense 1 comes from the story of Theseus in Greek mythology, who took a ball of thread into the labyrinth, paying it out on the way in and winding it up again to find his way out