"The accuser of our brethren," per Revelation (5)
I believe the answer is:
satan
'the accuser of our brethren per revelation' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for satan that I've seen before include "Apollyon" , "Prince of Darkness" , "Old Nick, Lucifer or Beelzebub" , "Fallen archangel" , "arch-fiend" .)