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It's very hard yet maths revision is essential (8)

Ross

I believe the answer is:

amethyst

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

'essential' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I don't see?

'it's very hard yet maths revision' is the wordplay.
'yet maths' can be anagrammed to 'AMETHYST'.
Though, I'm unsure how this is indicated.

'is' acts as a link.

Can you help me to learn more?

(Other definitions for amethyst that I've seen before include "rock" , "Type of quartz" , "Purple gem" , "February gemstone" , "Bluish-violet 3" .)

I've seen this clue in The Globe and Mail.
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