Historical era (5)
I believe the answer is:
epoch
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">historical era</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for epoch that I've seen before include "Period, era" , "Aeon" , "significant time" , "Long period of time" , "Significant period" .)