They oppose mostly red siblings (9)

I believe the answer is:

resisters

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'they oppose' is the definition.
The definition suggests an adverb but the answer is not.

'mostly red siblings' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'siblings' becomes 'sisters' (I've seen this before).
'red' with its final letter taken away is 're'.
're'+'sisters'='RESISTERS'

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

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