Nothing in key signature at first for singers (5)
I believe the answer is:
altos
'for singers' is the definition.
(alto is a kind of singer)
'nothing in key signature at first' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'in' means one lot of letters goes inside another.
'key' becomes 'alt' (the Alt keyboard button).
'at first' suggests taking the first letters.
The first letter of 'signature' is 's'.
'alt'+'s'='alts'
'o' placed within 'alts' is 'ALTOS'.
(Other definitions for altos that I've seen before include "Singers" , "Lowest female singing voices" , "Highest male, lowest female, voices" , "Females who sing in the lowest range" , "Choir members" .)