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