Upsetting competition for Perrier is not wise (5)

I believe the answer is:

naive

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'wise' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I don't see?

I cannot understand how the rest of the clue works.

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(Other definitions for naive that I've seen before include "Rather too innocent and trusting" , "Ingenous, easy to gull" , "Showing lack of experience or sophistication" , "Simple in style or behaviour" , "Lacking worldly experience" .)

I've seen this clue in The Times.

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