French king's eldest son (7)
I believe the answer is:
dauphin
'french king's eldest son' is the definition.
(I've seen this in another clue)
This is the entire clue.
(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" .)