Crossword Genius

Spirited grittiness, tough in the extreme (10)

Ross

I believe the answer is:

stringiest

I'm a little stuck... Click here to teach me more about this clue!

'in the extreme' is the definition.
I can't judge whether this definition defines the answer.

'spirited grittiness tough' is the wordplay.
'grittiness' can be anagrammed to 'STRINGIEST'.
But, I am not clear how the anagram is indicated.

Can you help me to learn more?

I've seen this clue in The Everyman puzzle in The Observer.
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