The most common and most likely answer to this crossword clue is the 9 letter word MANTICORE
I've seen this crossword clue in The Times
M
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Buddy explains!
The straight part
We know that "monster in myth" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case monster in myth = manticore.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 MANTICORE mean?
MANTICORE noun- A beast with the body of a lion (usually red), the tail of a scorpion, and the head/face of a man with a mouth filled with multiple rows of sharp teeth (like a shark), said to be able to shoot spikes from its tail or mane to paralyse prey. It may be horned, winged, or both; its voice is described as a mixture of pipes and trumpets.