Liquor store cashier uncovered slick breach of the law (7)
I believe the answer is:
'breach of the law' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for offence that I've seen before include "Breach of conduct" , "Transgression" , "The handler" , "Peccadillo" , "Breach of criminal law" .)