By receiving a service when you’re a reader (3)

I believe the answer is:

per

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'by' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

(Other definitions for per that I've seen before include "According to" , "Preposition indicating by means of" , "Preposition meaning for each" , "For each or every" .)

I've seen this clue in the Evening Standard.

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