Monarchy is lacking in common sense (5)

I believe the answer is:

realm

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

'monarchy' is the definition.
(I know that realm is a type of monarchy)

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

Can you help me to learn more?

(Other definitions for realm that I've seen before include "Country" , "Knowledge base" , "Kingdom, or field of activity" , "Area of influence" , "World" .)

I've seen this clue in The Irish Times.

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