Letting in, and letting on? (9)

I believe the answer is:

admission

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'letting' is the definition.
The definition and answer can be both related to property as well as being singular nouns.
Maybe they are linked in a way I don't understand?

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

Can you help me to learn more?

(Other definitions for admission that I've seen before include "acceptance" , "Entree" , "yield confession of sin maybe" , "for which one may be charged" , "concession" .)

I've seen this clue in The Sun.

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