Nieces seem excited adding number for theatrical set up (4,2,5)

I believe the answer is:

mise en scene

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

'set up' is the definition.
'mise-en-scene' can be an answer for 'set' (thesaurus). I'm not sure about the 'up' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Another definition for mise en scene that I've seen is " The setting of an event".)

I've seen this clue in The Irish Times.

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