A person of dangerous intelligence? (3)

I believe the answer is:

spy

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'a person' is the definition.
(spy is a kind of person)

I don't understand the rest of the clue.

(Other definitions for spy that I've seen before include "Person who secretly collects information as for government" , "One who collects secret information as for government" , "One gathers information surreptitiously" , "Undercover agent" , "Spot" .)

I've seen this clue in the Evening Standard.

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