Singers below mezzo-sopranos (5)
I believe the answer is:
altos
'singers below mezzo-sopranos' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for altos that I've seen before include "Females who sing in the lowest range" , "Lowest female singing voices" , "Highest male, lowest female, voices" , "High male or low female voices" , "Singers" .)