Singer in a long-forgotten oratorio (5)
I believe the answer is:
tenor
'singer' is the definition.
(I know that tenor is a type of singer)
'in a long-forgotten oratorio' is the wordplay.
'in' says the answer is hidden in the clue.
'TENOR' is hidden within 'long forgotten oratorio'.
(Other definitions for tenor that I've seen before include "intent" , "direction" , "Higher male voice" , "Carreras, for example" , "Big bell" .)