Ancient Persian king (6)
I believe the answer is:
xerxes
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">ancient persian king</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 xerxes that I've seen before include "Ancient Persian king" , "One old king" , "------I, son of Darius I, attempted invader of Greece, 480-79 BC" , "King of Persia, 486-465 BC" , "Name of ancient Persian kings" .)