Orchestra pit’s, surprisingly, extremely high (13)
I believe the answer is:
stratospheric
'extremely high' is the definition.
(the stratosphere is part of the atmosphere)
'orchestra pit's surprisingly' is the wordplay.
'surprisingly' indicates anagramming the letters.
'orchestra'+'pits'='orchestrapits'
'orchestrapits' anagrammed gives 'STRATOSPHERIC'.
(Other definitions for stratospheric that I've seen before include "Very high" , "Belonging to the upper air" , "Sky-high (informal)" .)