French king's eldest son (7)
I believe the answer is:
dauphin
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">french king's eldest son</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for dauphin that I've seen before include "Heir apparent to the French throne" , "French king's eldest son" , "Formerly, the eldest son of the King of France" , "French royal heir" , "King of France's heir" .)