Musical in need of opening song (3)
I believe the answer is:
air
'song' is the definition.
(air can mean a piece of music)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for air that I've seen before include "Publish" , "Ventilate, give voice to" , "Display" , "Song - atmosphere" , "Gas mixture required for breathing" .)