Forbiddance to act at the National Theatre (4)

I believe the answer is:

dont

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'forbiddance' is the definition.
Both the answer and definition are singular nouns.
Maybe there's an association between them I don't understand?

'act at the national theatre' is the wordplay.
'act' becomes 'do' (to do is to act or execute a task).
'at' says to put letters next to each other.
'the national theatre' becomes 'nt' (abbreviation).
'do'+'nt'='DONT'

'to' is the link.

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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