Crossword Genius

Nearly all these can be set in just under the counter (2,3,3)

Ross

I believe the answer is:

on the sly

'under the counter' is the definition.

I cannot understand how the remainder of the clue works.

(Other definitions for on the sly that I've seen before include "Surreptitiously" , "in private" , "With craft" , "Secretly or furtively" , "Under the table" .)

I've seen this clue in The Times.
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