Successor
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">successor</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that successor can be written as heir)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for heir that I've seen before include "One due to inherit" , "Treasured family possession" , "Expected successor" , "One hoping to succeed" , "One likely to succeed" .)