A watch for the arm (8)

I believe the answer is:

repeater

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'a watch' is the definition.
(thesaurus)

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

(Other definitions for repeater that I've seen before include "one feeding a duplicator" , "Timepiece that strikes the hours" , "habitual criminal" , "Handgun able to fire again without reloading" , "Maybe watch" .)

I've seen this clue in the Evening Standard.

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