Crossword Genius

Indefinite number in region's theatre (5)

Ross

I believe the answer is:

arena

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'theatre' is the definition.
(I've seen this before)

'indefinite number in region's' is the wordplay.
'indefinite number' becomes 'n' (mathematical symbol).
'in' is an insertion indicator.
'region's' becomes 'area' (I can't explain this - if you can you should believe this answer much more).
'n' put into 'area' is 'ARENA'.

Can you help me to learn more?

(Other definitions for arena that I've seen before include "Contest venue" , "dramatic situation" , "bowl? Possibly" , "where there's competition" , "Cheap Prom tickets" .)

I've seen this clue in The Telegraph.
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