Liquor store cashier uncovered slick breach of the law (7)

I believe the answer is:

offence

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

'breach of the law' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

(Other definitions for offence that I've seen before include "Breach of conduct" , "Transgression" , "The handler" , "Peccadillo" , "Breach of criminal law" .)

I've seen this clue in The Telegraph.

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