The highest adult male voice (4)
I believe the answer is:
alto
'the highest adult male voice' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for alto that I've seen before include "Singing voice between tenor and soprano" , "Voice part" , "Key old > male who scores high" , "Highest male voice: lowest female voice" , "One who sings" .)