Monarch's son (6)
I believe the answer is:
prince
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">monarch's son</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 prince that I've seen before include "He's a royal" , "Son of a royal house" , "See 1" , "Male heir to a throne" , "Canadian province" .)