Mounties, say, arresting ace thieves (7)
I believe the answer is:
raiders
'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" .)