A clue is a piece of evidence that helps solve a problem, or it's a slight hint. When you’re in the dark about the surprise party your friends are planning, they'll say, “She doesn’t have a clue!”
When you first join the police force, you’re mostly just walking the beat. But when you’re promoted to detective, your job is to look for that big clue that will help you solve the case. The original Middle English word was clew, which meant “a ball of thread.” If you were trapped in a labyrinth, you could follow the clew to help find your way out.