Choir singers (6)
'choir singers' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for altos that I've seen before include "High male or low female voices" , "Lowest female singing voices" , "Females who sing in the lowest range" , "Highest male, lowest female, voices" , "Choir members" .)
(Other definitions for tenors that I've seen before include "Male singing voices" , "Male singers with high register" , "Opera singers" , "Male singers from Reston" .)