One's rehabilitated a defector to Labour? (2-3)
I believe the answer is:
ex-con
'one's rehabilitated' is the definition.
(someone rehabilitated in prison)
I don't understand the rest of the clue.
(Other definitions for ex-con that I've seen before include "Person once inside, now outside" , "Old lag" , "She used to serve porridge" , "One who has served time" , "One now free" .)