Traitor in fraternity (3)
I believe the answer is:
rat
'traitor' is the definition.
(rat can mean a treacherous person)
'in fraternity' is the wordplay.
'in' indicates a hidden word.
'RAT' is hidden within 'fraternity'.
(Other definitions for rat that I've seen before include "Renegade" , "Cad" , "Unpopular rodent" , "Rodent; informer (colloq.)" , "Scumbag" .)