Voice is key in brief authority (9)
I believe the answer is:
contralto
'voice' is the definition.
(contralto is a kind of voice)
'key in brief authority' is the wordplay.
I cannot really see how this works, but
'key' could be 'alt' (the Alt keyboard button) and 'alt' is located in the answer.
This explanation may well be incorrect...
'is' is the link.
Can you help me to learn more?
(Other definitions for contralto that I've seen before include "Lowest range for a woman singer" , "Female voice of the lower register." , "Low female voice" , "Lowest female singing voice" , "Lowest female singer" .)