Plain stage in auditorium (6)
I believe the answer is:
steppe
'plain' is the definition.
(I know that steppe is a type of plain)
'stage in auditorium' is the wordplay.
'stage' becomes 'step' (as in a step or stage of a process).
'in auditorium' shows a homophone (sound like) (as the word might sound to an audience).
'step' is a homophone of 'STEPPE'.
(Other definitions for steppe that I've seen before include "Dry plain of Eurasia" , "Edxtensive plain" , "(Russian) grassy plain" , "Treeless plain of Russia" , "plain territory" .)