Melody, air (4)
I believe the answer is:
tune
I believe this is a double definition.
'melody' is the first definition.
(I know that melody can be written as tune)
'air' is the second definition.
(air can mean a piece of music)
(Other definitions for tune that I've seen before include "Air, melody" , "Song" , "Improve quality" , "Melody; adjust (engine)" , "Adjust (car engine); air" .)