Thief goes to the gym regularly, perhaps (6)

I believe the answer is:

lifter

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'thief' is the definition.
(I've seen this in another clue)

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

(Other definitions for lifter that I've seen before include "One who raises things like weights" , "One putting up" .)

I've seen this clue in The Irish Times.

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