Sly, cunning (6)
I believe the answer is:
sneaky
This clue is a double definition.
'sly' is the first definition.
(I've seen this in another clue)
'cunning' is the second definition.
(I've seen this before)
(Other definitions for sneaky that I've seen before include "Cunning" , "Furtive or given to telling tales" , "Furtive, lacking candour" , "Sly and devious" , "Deceitful, underhand" .)