Tenor's last note upset acoustic device (6)
I believe the answer is:
reverb
'acoustic device' is the definition.
(type of acoustic effect)
'tenor's last note upset' is the wordplay.
'last' says to take the final letters.
'note' becomes 'breve' (musical note).
'upset' says the letters should be written in reverse.
The last letter of 'tenor' is 'r'.
'breve' written backwards gives 'everb'.
'r'+'everb'='REVERB'
(Other definitions for reverb that I've seen before include "Echo in music" , "Echoing device" , "Echo effect applied to sound" , "Electronic effect added to music" , "Musical device producing echoes" .)