Information usually of service (3)

I believe the answer is:

gen

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'information' is the definition.
(gen can informally mean information)

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

Can you help me to learn more?

(Other definitions for gen that I've seen before include "Informal information" , "Info" , "Information (colloq.)" , "opening book, in short" , "Information yielded" .)

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