Rule: ignore changes when nought is missing (5)

I believe the answer is:

reign

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

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

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

Can you help me to learn more?

(Other definitions for reign that I've seen before include "Period on the throne" , "Rule of monarch" , "Niger (anag)" , "Dominion" , "Monarch's time in the job" .)

I've seen this clue in The Times.

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