First-class introduction to rousing song (3)
I believe the answer is:
Here is my best explanation:
'song' is the definition.
'first-class introduction to rousing' is the subsidiary indication.
'first class' becomes 'ai' (A1 - top quality).
'introduction' indicates taking the first letters.
The initial letter of 'rousing' is 'r'.
(Other definitions for air that I've seen before include "Strain", "Tune; mien", "Show", "what emerges from singer's lungs?", "Atmosphere surrounding us", "Demeanour", "Impression", "Simple tune", "Skye Boat Song, perhaps" and "Look - warm and dry".)