In law, some allegations can be doubtful (5)

I believe the answer is:

legal

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'in' is the definition.
Both the definition and answer are adjectives. Maybe you can see a link between them that I can't see?

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for legal that I've seen before include "Compliant with the law" , "Allowed by law" , "not banned" , "It's acceptable" , "Conforming with the law" .)

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