Watch your heart! (6)

I believe the answer is:

ticker

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'watch' is the definition.
(synonyms)

I cannot understand how the rest of the clue works.

(Other definitions for ticker that I've seen before include "teacher giving correct marks" , "- - -tape machine" , "Telegraph instrument" , "Hunter, perhaps" , "Old watch or informal heart" .)

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

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