The most common and most likely answer to this crossword clue is the 8 letter word EUGENICS
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The straight part
We know that "Stock improvement" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case stock improvement = eugenics.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 EUGENICS mean?
EUGENICS noun- A social philosophy or practice which advocates the improvement of human hereditary qualities through selective breeding, either by encouraging people with good genetic qualities to reproduce (positive eugenics), or discouraging people with bad genetic qualities from reproducing (negative eugenics), or by technological means.