Flashy auteur warned those in the middle to keep toward the rear (6)

I believe the answer is:

astern

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'toward the rear' is the definition.
(I've seen this in another clue)

I can't explain the remainder of the clue.

(Other definitions for astern that I've seen before include "Backward" , "following behind vessel" , "Behind (naut.)" , "Towards a ship's rear" , "Aft" .)

I've seen this clue in The Irish Times.

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