Only one customer's requirement in a series (5)

I believe the answer is:

order

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This clue could be a double definition.

'only one customer's requirement' is the first definition.
'order' can be an answer for 'requirement' (thesaurus). I am unsure of the 'only one customer's' bit.

'series' is the second definition.
(thesaurus)

'in a' is the link.

Can you help me to learn more?

(Other definitions for order that I've seen before include "Instruct" , "peace" , "Command, mandate" , "Decree" , "Command; neatness" .)

I've seen this clue in the Evening Standard.

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