Simple tune (3)
'simple tune' is the definition.
(air can mean a piece of music)
This is the entire clue.
(Other definitions for air that I've seen before include "Ventilate or give expression to" , "Atmosphere surrounding us" , "Ventilate with breathing gas" , "Mixture of gases required for breathing" , "Tune - mood - atmosphere" .)
'simple tune' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ditty that I've seen before include "Air" , "Little tune" , "Light-hearted song" , "Simple tune" , "Short simple song" .)