Fast one (4)
I believe the answer is:
scam
'fast one' 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 "'A con, deception (4)'" , "Swindle (Slang)" , "ploy" , "Fraudulent trick or swindle" , "Confidence trick, swindle" .)