Vocal power (6)
I believe the answer is:
speech
I believe this clue is a double definition.
'vocal' is the first definition.
(thesaurus)
'power' is the second definition.
(speech is a kind of power)
(Other definitions for speech that I've seen before include "Talk given to an audience" , "Harangue" , "Formal talk to an audience" , "Communication by word of mouth" , "Oration or talk" .)