Key thus backfiring for singers (5)
I believe the answer is:
altos
'singers' is the definition.
(alto is a kind of singer)
'key thus backfiring' is the wordplay.
'key' becomes 'alt' (the Alt keyboard button).
'thus' becomes 'so' (synonyms).
'backfiring' says the letters should be written backwards.
'so' back-to-front is 'os'.
'alt'+'os'='ALTOS'
'for' is the link.
(Other definitions for altos that I've seen before include "Lowest female singing voices" , "Singers" , "Females who sing in the lowest range" , "High male or low female voices" , "Highest male, lowest female, voices" .)