Air or tune (6)
I believe the answer is:
melody
I believe this is a double definition.
'air' is the first definition.
(I know that air can be written as melody)
'tune' is the second definition.
(I know that tune can be written as melody)
(Other definitions for melody that I've seen before include "It forms the core of a tune" , "Musical line" , "Refrain" , "Strain" , "Harmonious tune or song" .)