Services offload fourth and sixth in sequence (6)

I believe the answer is:

series

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'sequence' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for series that I've seen before include "Sequence of programmes etc." , "Set of TV programmes" , "Sequence; tone row" , "Set of anything" , "Sets in sequence" .)

I've seen this clue in The Irish Times.

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