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 a hidden word.
'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?" .)