Dukedom (5)
I believe the answer is:
duchy
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">dukedom</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for duchy that I've seen before include "Duke's domain" , "Cornwall, Lancaster such a territory" , "Territory of a nobleman - Cornwall, for instance" , "Duke's territory" , "Aristocrat's territory" .)