The most common and most likely answer to this crossword clue is the 9 letter word VAMPIRISM
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Buddy explains!

The straight part
We know that "sucking neck voraciously" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sucking neck voraciously = vampirism.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 VAMPIRISM mean?
VAMPIRISM noun- Systemic lupus erythematosus (autoimmune disease)
- The state of being a vampire
- Practices associated with vampires, in particular blood-drinking and the draining of a victim's life-force.
- Extortion