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 the answer is hidden within the clue.
'RAT' can be found hidden inside 'fraternity'.
(Other definitions for rat that I've seen before include "Renegade" , "Cad" , "Unpopular rodent" , "Rodent; informer (colloq.)" , "Scumbag" .)