Lively musical piece (7)
I believe the answer is:
scherzo
'lively musical piece' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for scherzo that I've seen before include "Fast movement (mus.)" , "Fast movement of symphony" , "Vigorous musical movement" , "Fast movement (music)" , "Form in music" .)