It's behind atrocities these Russian extremists create (6)
I believe the answer is:
astern
'it's behind' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
(Other definitions for astern that I've seen before include "Towards the rear of a ship" , "Backward" , "following behind vessel" , "Towards the back (of a ship)" , "Rearward" .)