First-class resistance song (3)
I believe the answer is:
air
'song' is the definition.
(thesaurus)
'first-class resistance' is the wordplay.
'first class' becomes 'ai' (A1 - top quality).
'resistance' becomes 'r' (abbreviation).
'ai'+'r'='AIR'
(Other definitions for air that I've seen before include "Cool" , "Element; tune" , "Lay" , "Atmosphere surrounding us" , "Atmosphere (of a melody?)" .)