A strand of yarn etc. as used to guide one through a labyrinth; something which points the way, a guide.
Information which may lead one to a certain point or conclusion.
An object or a kind of indication which may be used as evidence.
Insight or understanding ("to have a clue [about]" or "to have clue". See have a clue, clue stick)
To provide with a clue.
To provide someone with information which he or she lacks (often used with "in" or "up").