The air in the opera house? (4)
I believe the answer is:
aria
'the air' is the definition.
(thesaurus)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for aria that I've seen before include "Musical performance" , "Operatic number" , "Puccini item" , "Vocal solo in a cantata etc" , "Opera solo" .)