Surreptitiously (2,3,3)
I believe the answer is:
on the sly
'surreptitiously' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for on the sly that I've seen before include "With craft" , "Behind someone's back" , "Secretly or furtively" , "Under the table" , "in private" .)