Traitor in the fraternity (3)
I believe the answer is:
rat
'traitor' is the definition.
(thesaurus)
'in the fraternity' is the wordplay.
'in' indicates the answer is hidden within the clue.
'RAT' is hidden within 'fraternity'.
(Other definitions for rat that I've seen before include "one not going down with ship" , "Unpleasant character" , "Renegade" , "Rodent - deserter" , "Contemptible person" .)