Prone to scheming (3)
I believe the answer is:
sly
'prone to scheming' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sly that I've seen before include "'Cunning, wily (3)'" , "Devious and furtive" , "Unpleasantly cunning and secretive" , "Wily, shifty" , "Cunning, deceitful" .)