Loud speaker, improvisation of tenor underway? (7)
I believe the answer is:
stentor
'loud speaker' is the definition.
(I've seen this in another clue)
I can't explain the remainder of the clue.
(Other definitions for stentor that I've seen before include "Loud-voiced Greek at Troy" , "Mythical Greek warrior with an unusually loud voice" , "man with loudest voice" , "powerful speaker" .)