Empire (5)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">empire</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 realm that I've seen before include "Royal domain" , "Kingdom; field of interest" , "Kingdom, domain" , "King's domain" , "Area of interest/activity" .)