Nullify rundown region (4)

I believe the answer is:

undo

I'm a little stuck... Click here to teach me more about this clue!

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

'rundown region' is the wordplay.
'UNDO' is hidden within the letters.
However, I am unsure how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for undo that I've seen before include "Unfasten - ruin" , "set free" , "Bring to ruin" , "Ruin or loosen ties" , "Cancel, annul; open" .)

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

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