The most common and most likely answer to this crossword clue is the 8 letter word CHIMAERA
I've seen this crossword clue in The Guardian
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Buddy explains!

The straight part
We know that "female monster" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case female monster = chimaera.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 CHIMAERA mean?
CHIMAERA noun- A cartilaginous marine fish in the subclass Holocephali and especially the order Chimaeriformes, with a blunt snout, long tail, and a spine before the first dorsal fin
- Alternative letter-case form of Chimera (a flame-spewing monster often represented as having two heads, one of a goat and the other of a lion; the body of a goat; and a serpent as a tail).
- Any fantastic creature with parts from different animals.
- Anything composed of very disparate parts.
- A foolish, incongruous, or vain thought or product of the imagination.
- A grotesque like a gargoyle, but without a spout for rainwater.
- An organism with genetically distinct cells originating from two or more zygotes.
- Usually chimaera: a cartilaginous marine fish in the subclass Holocephali and especially the order Chimaeriformes, with a blunt snout, long tail, and a spine before the first dorsal fin.