It sounds highly unnatural (8)
I believe the answer is:
falsetto
'it sounds highly unnatural' is the definition.
The definition suggests a singular noun which matches the answer.
This is the entire clue.
Can you help me to learn more?
(Other definitions for falsetto that I've seen before include "High singing method" , "Voice one's raised" , "Aritifical upper register of male voice" , "High-pitched male voice" , "Unusually high voice" .)