Crossword Genius

Mounties, say, arresting ace thieves (7)

Ross

I believe the answer is:

raiders

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'thieves' is the definition.
(raider is a kind of thief)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for raiders that I've seen before include "' - - of the Lost Ark'" , "Attacking force" , "Marauders" , "Marauding attackers" .)

I've seen this clue in The Times.
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