Often, oratorios employ one (5)
I believe the answer is:
tenor
'one' is the definition.
Both the answer and definition are adjectives. Maybe you can see a link between them that I can't see?
'often oratorios employ' is the wordplay.
'employ' indicates the answer is hidden within the clue (I've seen 'employing' mean this (the answer is used or employed by the clue)).
'TENOR' can be found hidden inside 'often oratorios'.
Can you help me to learn more?
(Other definitions for tenor that I've seen before include "Male singer with high register" , "Caruso or Domingo" , "High male singing voice like Pavarotti" , "Higher male voice" , "High voice - low bell" .)