clue
[N]ключ к разгадке; ключ ; нить ; ход мыслей; улика ; информация

As far as his destination was concerned, the clue had to be in the dedication.
The heading of this chapter gives you a clue.
That's the important clue.
The leaves had distinct short stalks, a clue that it was a sessile oak.
It just seemed weird to me that we didn't have a clue about the line-up.