Be briefly doubtful of what might be detected (3)

I believe the answer is:

sus

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'be' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

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

Can you help me to learn more?

(Other definitions for sus that I've seen before include "Something police" , "Suspicion (slang)" , "Suspicion (of a crime)" , "One interesting the police" , "Believe guilty (sl.)" .)

I've seen this clue in the Evening Standard.

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