One keeping watch? (5)

I believe the answer is:

strap

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'one keeping watch?' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

This is all the clue.

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(Other definitions for strap that I've seen before include "e.g. Belt" , "Leather fastening" , "Shoulder band" , "Narrow strip of leather" , "something to hang on to" .)

I've seen this clue in the Evening Standard.

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