The most common and most likely answer to this crossword clue is the 10 letter word ADAMANTINE
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The straight part
We know that "Extremely hard" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case extremely hard = adamantine.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 ADAMANTINE mean?
ADAMANTINE adjective- Made of adamant, or having the qualities of adamant; incapable of being broken, dissolved, or penetrated.
- Like the diamond in hardness or luster.