Eldest son of French king (7)
I believe the answer is:
dauphin
'eldest son of french king' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for dauphin that I've seen before include "Heir apparent to the French throne" , "former royal" , "French royal heir" , "King of France's heir" , "Eldest son of the French king" .)