Those who remain so are not taken in (7)

I believe the answer is:

outside

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

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

Can you help me to learn more?

(Other definitions for outside that I've seen before include "Al fresco" , "Not indoors" , "In the fresh air?" , "Without" , "Tedious (anag)" .)

I've seen this clue in The Globe and Mail.

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