Fraudulent activity (4)
I believe the answer is:
scam
'fraudulent activity' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for scam that I've seen before include "Swindle (Slang)" , "An informal swindle" , "Screw" , "A slangy swindle or fraud" , "Fraudulent business scheme" .)