Person singing some forgotten oratorio (5)
I believe the answer is:
tenor
'person singing' is the definition.
(type of singing voice)
'some forgotten oratorio' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'TENOR' is hidden in the letters of 'forgotten oratorio'.
(Other definitions for tenor that I've seen before include "Drift of thought" , "Carreras, for example" , "Big bell" , "Pitch range" , "exact words" .)