The most common and most likely answer to this crossword clue is the 9 letter word UNKINDEST
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The straight part
We know that "Extremely cruel" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case extremely cruel = unkindest.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 UNKINDEST mean?
UNKINDEST adjective- Lacking kindness, sympathy, benevolence, gratitude, or similar; cruel, harsh or unjust; ungrateful.
- Not kind; contrary to nature or type; unnatural.
- Having no race or kindred; childless.