Traitor within fraternity (3)
I believe the answer is:
rat
'traitor' is the definition.
(rat can mean a treacherous person)
'within fraternity' is the wordplay.
'within' 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" , "person doing shopping" , "Abandon" , "Scumbag" , "one who deserts the sinking ship?" .)