One after another by a rotary system? (2,4)

I believe the answer is:

in turn

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'one after another' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

(Other definitions for in turn that I've seen before include "One after another" , "in an orderly fashion" , "orderly treatment" , "not out of place" , "a succession of things happen" .)

I've seen this clue in the Evening Standard.

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