First-class introduction to rousing song (3)
I believe the answer is:
air
'song' is the definition.
(thesaurus)
'first-class introduction to rousing' is the wordplay.
'first class' becomes 'ai' (A1 - top quality).
'introduction to' suggests taking the first letters (I've seen 'introductions to' mean this).
The first letter of 'rousing' is 'r'.
'ai'+'r'='AIR'
(Other definitions for air that I've seen before include "Light wind; song" , "Cool" , "Atmosphere, manner" , "Ventilate with breathing gas" , "in which birds fly around" .)