Crossword Genius

One opposing extraordinary action, Conservative must go (4)

Ross

I believe the answer is:

anti

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'one opposing' is the definition.
(prefix meaning 'opposing')

I don't understand how the rest of the clue works.

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(Other definitions for anti that I've seen before include "One who is against (something)" , "Hostile" , "'No' voter" , "one opposing?" , "Against, informally" .)

I've seen this clue in The Times.
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