The most common and most likely answer to this crossword clue is the 7 letter word DAUPHIN
I've seen this crossword clue in Independent
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More clues leading to the result DAUPHIN
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The cryptic/wordplay part
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What does DAUPHIN mean?
DAUPHIN noun- The eldest son of the king of France. Under the Valois and Bourbon dynasties, the Dauphin of France, generally shortened to Dauphin, was heir apparent to the throne of France. The title derived from the main title of the Dauphin, Dauphin of Viennois.
- Allegorical An eldest son.