Unnaturally high voice (8)
I believe the answer is:
falsetto
'unnaturally high voice' 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 "Abnormally high voice" , "Artificially high singing voice" , "Artificial pitch" , "High vocal range" , "Unnaturally high voice" .)