Fraudulent operation (4)
I believe the answer is:
scam
'fraudulent operation' 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 "Rip off" , "Swindle, fraud (colloq.)" , "Confidence trick, swindle" , "Fraudulent action" , "Fraudulent business scheme" .)