Criminal sent to catch another one, so it is said (5)

I believe the answer is:

thief

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'criminal' is the definition.
(I know that criminal can be written as thief)

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

Can you help me to learn more?

(Other definitions for thief that I've seen before include "A robber or pickpocket" , "Unlawful taker" , "He takes" , "Stealer" , "Person who steals things" .)

I've seen this clue in The Telegraph.

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