The most common and most likely answer to this crossword clue is the 10 letter word RINDERPEST
I've seen this crossword clue in Independent
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The straight part
We know that "condition of stock" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case condition of stock = rinderpest.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.
What does RINDERPEST mean?
RINDERPEST noun- A contagious disease of ruminants and swine caused by Rinderpest virus, an RNA virus of the genus Morbillivirus.