Something to hang on to, keeping watch? (5)

I believe the answer is:

strap

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'something to hang on to' is the definition.
(I've seen this before)

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

Can you help me to learn more?

(Other definitions for strap that I've seen before include "loopy band" , "something to hang on to" , "e.g. Belt" , "Beat; securing band" , "Shoulder band" .)

I've seen this clue in the Evening Standard.

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