The most common and most likely answer to this crossword clue is the 10 letter word COCKATRICE
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More clues leading to the result COCKATRICE
Buddy explains!

The straight part
We know that "Monster" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case monster = cockatrice.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 COCKATRICE mean?
COCKATRICE noun- A legendary creature about the size and shape of a dragon or wyvern, but in appearance resembling a giant rooster, with some lizard-like characteristics.
- Mistress, harlot.
- A snake or serpent that appears to be hatched of a rooster, or cock's, egg.
- (speculated) The cobra.
- Any venomous or deadly thing.