Singer into vocal tone (4)
I believe the answer is:
alto
'singer' is the definition.
(alto is a kind of singer)
'into vocal tone' is the wordplay.
'into' indicates a hidden word.
'ALTO' is hidden within 'vocal tone'.
(Other definitions for alto that I've seen before include "The highest adult male singing-voice" , "Member of choir" , "Highest male voice: lowest female voice" , "Male singer" , "The highest adult male singing voice" .)