Crossword Genius

In theatre, the really old number (5)

Ross

I believe the answer is:

ether

'old number' is the definition.
(chemical historically used to numb patients)

'in theatre the really' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ETHER' is hidden within 'theatre the really'.

(Other definitions for ether that I've seen before include "Medium supposedly" , "Colourless, highly flammable liquid" , "Anaesthetic; clear sky (poetic)" , "Highly flammable liquid once used as an anaesthetic" , "It could knock you out" .)

I've seen this clue in The Sun.
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