Crossword Genius

It's behind atrocities these Russian extremists create (6)

Ross

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" .)

I've seen this clue in The Guardian.
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