A shopkeeper with whom no secret is safe? (8)

I believe the answer is:

retailer

I'm a little stuck... Click here to teach me more about this clue!

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

I don't understand the rest of the clue.

(Other definitions for retailer that I've seen before include "Store-keeper" , "Shop" , "Tradesman" , "One who sells goods to the geneal public" , "One selling to public" .)

I've seen this clue in the Evening Standard.

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