Choir member (4)
'choir member' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for alto that I've seen before include "Key old > male who scores high" , "Singer's hidden inside chemical toilet" , "A singing voice or pitch" , "Low female singing voice" , "Lowest female singing voice" .)
(Other definitions for bass that I've seen before include "Large fish - singer" , "Variety of sea and freshwater perch" , "Fish; vocalist" , "Type of fish" , "deep in the sound" .)