Bill's in this as the spokesman (5)
I believe the answer is:
voice
'the spokesman' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for voice that I've seen before include "Sound of speaker" , "Give expression to" , "eg Bass" , "right to have a say" , "Ability to speak, sing" .)