Hunter in oratorio, Nimrod (5)
I believe the answer is:
orion
'hunter' is the definition.
(I've seen this before)
'in oratorio nimrod' is the wordplay.
'in' indicates a hidden word.
'ORION' can be found hidden inside 'oratorio nimrod'.
(Other definitions for orion that I've seen before include "He hunted" , "Great hunter of Greek myth, and a constellation" , "A giant hunter in Greek legend, and a constellation" , "Prominent constellation" , "Star" .)