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