Long period of history (3)
I believe the answer is:
era
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">long period of history</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for era that I've seen before include "Special time" , "Stage" , "Long historic period" , "quite some time" , "Geological period" .)