Acting dishonestly to gain advantage (8)
I believe the answer is:
cheating
'acting dishonestly to gain advantage' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cheating that I've seen before include "Acting dishonestly to gain advantage" , "Swindling or deceiving" , "Swindling or defrauding" , "Acting unfairly" , "Cleverly avoiding" .)