Unnatural high (8)
I believe the answer is:
'unnatural high' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for falsetto that I've seen before include "Artificial pitch" , "Artificially high voice pitch" , "Unnaturally high-pitched voice" , "Note production" , "Artificially high voice of latest sort" .)