Minder to keep watch (5)

I believe the answer is:

guard

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'watch' is the definition.
(I know that guard is a more specific form of the action watch)

I don't understand the rest of the clue.

(Other definitions for guard that I've seen before include "Shield" , "Police" , "Protect(or)" , "Protect" , "Sentinel" .)

I've seen this clue in The Sun.

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