Prehistoric period (4,3)
I believe the answer is:
iron age
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">prehistoric period</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 iron age that I've seen before include "historical period" , "Period when metal tools were developed" , "Very old period" , "Archaeological period" , "prehistoric times" .)