Clew meant a ball of thread in Medieval England.

Now clew or clue denotes a guide to the solution to a difficult problem.

What is the connection between clew/clue and a solution to a problem?

In a Greek legend Theseus kills the Minotaur of Crete in a labyrinth.

He uses a ball of thread to find his way out of the labyrinth.

The ball of thread ‘clew’ has changed to the ‘clue’-

the guide to the solution of a difficult problem.