Vigilant horologist (8)

I believe the answer is:

watchman

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'vigilant horologist' is the definition.
The definition suggests a singular noun which matches the answer.

This is all the clue.

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(Other definitions for watchman that I've seen before include "Security guard" , "Sentinel" , "'Go Set a --', Harper Lee novel" , "fellow on duty?" , "Night guard" .)

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

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