Corrupt imprisoned politician expelled, losing nothing, is one in the know (7)
I believe the answer is:
insider
'is one in the know' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for insider that I've seen before include "one's not allowed to trade" , "in prison?" , "Wrong sort of trading" , "member of the club" , "Doing time, right, for this" .)