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