Deceive; disadvantage (3)
I believe the answer is:
con
This clue is a double definition.
'deceive' is the first definition.
(I've seen this in another clue)
'disadvantage' is the second definition.
(as in pros and cons)
(Other definitions for con that I've seen before include "Swindle or see" , "Scam, swindle" , "Opposed" , "Prisoner (sl.)" , "Deceive; disadvantage" .)