...saying top celebrities are just so extreme (10)

I believe the answer is:

maximalist

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'just so extreme' is the definition.
The definition suggests a singular noun which matches the answer.

'saying top celebrities' is the wordplay.
'saying' becomes 'maxim' (maxim is a kind of saying).
'top celebrities' becomes 'alist' (the A-list means very famous people).
'maxim'+'alist'='MAXIMALIST'

'are' acts as a link.

Can you help me to learn more?

I've seen this clue in The Irish Times.

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