Crossword Genius

Large room, one we organised to show scary movie (9)

Ross

I believe the answer is:

hallowe'en

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

'show scary movie' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

'large room one we organised' is the wordplay.
'large room' becomes 'hall' (a hall is a large room).
'organised' is an anagram indicator.
'one'+'we'='onewe'
'onewe' is an anagram of 'oween'.
'hall'+'oween'='HALLOWEEN'

'to' is the link.

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I've seen this clue in The Telegraph.
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