Jaunty musical rhythm (4)
I believe the answer is:
lilt
'jaunty musical rhythm' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for lilt that I've seen before include "Tune with light pleasant rhythm" , "Swing - music" , "Jaunty musical rhythm" , "Light, bouncy rhythm" , "...jaunty rhythm, perhaps" .)