Arm for an armed hold-up (5)

I believe the answer is:

sling

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'arm' is the definition.
(I know that sling is a type of arm)

I can't explain the remainder of the clue.

(Other definitions for sling that I've seen before include "harness for broken arm" , "Keeping afloat" , "Bandage support; hurl" , "Catapult - shoe - support" , "shy" .)

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

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