Lass doing wild flourish on the harp (9)
I believe the answer is:
glissando
'flourish on the harp' is the definition.
'glissando' can be an answer for 'flourish' (I have seen 'Musical flourish' mean 'glissando' so perhaps 'flourish' could also mean 'glissando'). I'm unsure of the 'on the harp' bit.
'lass doing wild' is the wordplay.
'wild' indicates an anagram.
'lass'+'doing'='lassdoing'
'lassdoing' is an anagram of 'GLISSANDO'.
Can you help me to learn more?
(Other definitions for glissando that I've seen before include "Slide between two notes" , "Note with sliding pitch" , "Musical flourish" , "trombone slide, for example" , "Sliding effect in moving from one musical note to another" .)