Fast one (4)
I believe the answer is:
scam
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fast one</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for scam that I've seen before include "'A con, deception (4)'" , "Swindle (Slang)" , "ploy" , "Fraudulent trick or swindle" , "Confidence trick, swindle" .)