Son of Agamemnon (7)
I believe the answer is:
'son of agamemnon' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for orestes that I've seen before include "figure in Greek tragedy" , "Matricide" , "Son of Agamemnon" , "he killed his mother and her lover" , "Tragically, killer" .)