Illegal operation (4)
I believe the answer is:
scam
'illegal operation' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for scam that I've seen before include "Fraudulent trick or swindle" , "Fraud, trick (slang)" , "A slangy swindle or fraud" , "Fraud, dishonest scheme" , "fiddle" .)