Upstart in female attire backing sentry (5)

I believe the answer is:

guard

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'sentry' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for guard that I've seen before include "Custodian" , "See 1" , "Yeomen" , "Protect(or)" , "Sentinel" .)

I've seen this clue in the Sydney Morning Herald.

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