Deserter from a concentration camp (3)
I believe the answer is:
rat
'deserter' is the definition.
(both can mean a traitor)
'from a concentration camp' is the wordplay.
'from' indicates the answer is hidden within the clue.
'RAT' is hidden in the letters of 'concentration camp'.
(Other definitions for rat that I've seen before include "Contemptible person" , "Plague-carrying rodent" , "Rodent; traitor" , "Scumbag" , "Renegade" .)