Campaign launch in bar? (7)

I believe the answer is:

prevent

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'bar?' is the definition.
(I know that bar is a more specific form of the action block)

I cannot understand how the rest of the clue works.

(Other definitions for prevent that I've seen before include "Obviate" , "Keep from arising" , "Keep from happening" , "Stop, avert" , "Bar" .)

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

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