The most common and most likely answer to this crossword clue is the 12 letter word BLOODSUCKERS
I've seen this crossword clue in The Daily Telegraph
B
L
O
O
D
S
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C
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Buddy explains!

The straight part
We know that "extortionists" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case extortionists = bloodsuckers.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 BLOODSUCKERS mean?
BLOODSUCKERS noun- An animal that drinks the blood of others, especially by sucking blood through a puncture wound; a hemovore.
- (by extension) Any parasite.
- (by extension) One who attempts to take as much from others as possible; a leech.
- A vampire.