Working on the sly? Really! (8)
I believe the answer is:
honestly
'really' is the definition.
(I've seen this before)
'working on the sly?' is the wordplay.
'working' is an anagram indicator.
'on'+'the'+'sly'='onthesly'
'onthesly' with letters rearranged gives 'HONESTLY'.
(Other definitions for honestly that I've seen before include "How law-abiding people act" , "On the sly (anag.)" , "I assure you!" , "No kidding!" , "Frankly -- that's outrageous!" .)