Highest adult male voice (4)
I believe the answer is:
alto
'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 "Voice part" , "Singer's hidden inside chemical toilet" , "The highest adult male singing-voice" , "Singing voice between tenor and soprano" , "Key old > male who scores high" .)