The correct answer to this open question is the following.
Although you did not include further reference or context, we can answer is general terms.
Historian's clues are similar to investigators' clues. Historians have to follow these clues to collect information to investigate dates, characters, motives, numbers, and other facts that were important parts in a historic event or incident. Not everything is there, on the surface for the historians to grab it. No, most of the time, historians, as well as archeologists or anthropologists, have to follow clues to do some research in one direction or the other. That is why they go deeper to find the truth.
