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' says the answer is hidden in 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?" .)