As I was typing this, Bingster beat me to it. :-) Had this been a few days ago, I would have agreed with you, that "clew" was a misteak.
However, incredibly coincidentally, last Sunday I read an episode of the old "Tom Swift" books, entitled "Tom Swift and His Photo Telephone". It was written in 1914.
As you might know, Tom is an inventor and amateur detective. In this book he uses the word "clew" several times, in the context you'd expect, a piece of evidence or information used in the detection of a crime or solving of a mystery.
I looked it up, "clew" is indeed an archaic variant spelling for "clue".
It comes from the Dutch "kluwen", a ball of string. Unravelling a ball of string can help you find you way back out of a maze, clues, as it were, to help you backtrack.