Act dishonestly to gain an advantage (5)
I believe the answer is:
cheat
'act dishonestly to gain an advantage' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for cheat that I've seen before include "One dealing dishonestly" , "offending pupil" , "Fool" , "Teach? (Anag)" , "Hoodwink" .)