The most common and most likely answer to this crossword clue is the 12 letter word TWO-YEAR-OLD
I've seen this crossword clue in The Guardian
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W
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Buddy explains!
The straight part
We know that "Thrower of tantrums, perhaps/Thrower of tantrums/, /perhaps/one that’s ridden in 10 Queen Mary Stakes" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case thrower of tantrums, perhaps/thrower of tantrums/, /perhaps/one that’s ridden in 10 queen mary stakes = two-year-old.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.