Not a silent thief (7)

I believe the answer is:

rustler

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'thief' is the definition.
(I know that rustler is a type of thief)

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

Can you help me to learn more?

(Other definitions for rustler that I've seen before include "Cattle stealer" , "Stock taker?" , "He stole cattle in the old west" , "Cattle-thief" , "person taking stock" .)

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

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