Choir members in cathedral to sing (5)
I believe the answer is:
altos
'choir members' is the definition.
(I've seen this before)
'in cathedral to sing' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ALTOS' is hidden in the letters of 'cathedral to sing'.
(Other definitions for altos that I've seen before include "Females who sing in the lowest range" , "High male or low female voices" , "Choir members" , "Lowest female singing voices" , "Singers" .)