An air of simplicity (5)
I believe the answer is:
ditty
'an air' is the definition.
(I've seen this before)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for ditty that I've seen before include "Short simple song" , "Simple tune" , "Air" , "Little tune" , "Light-hearted song" .)