It's very hard yet maths revision is essential (8)
I believe the answer is:
amethyst
'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" .)