Trendy nuns, they're very assertive (9)

I believe the answer is:

insisters

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'very assertive' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'trendy nuns' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'nuns' becomes 'sisters' (nuns often use the title 'sister').
'in'+'sisters'='INSISTERS'

'they're' acts as a link.

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I've seen this clue in the Sydney Morning Herald.

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