The most common and most likely answer to this crossword clue is the 9 letter word WELL-BRED
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More clues leading to the result WELL-BRED
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The straight part
We know that "of good stock" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case of good stock = well-bred.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
Crossword Answer definitions
WELL-BRED adjective- (of animals) Of good breed.
- (by extension) Well-mannered and refined because of a good upbringing and education.