Corrupt senator in betrayal (7)
I believe the answer is:
treason
'betrayal' is the definition.
(I know that betrayal can be written as treason)
'corrupt senator' is the wordplay.
'corrupt' indicates anagramming the letters (a corrupted form).
'senator' is an anagram of 'TREASON'.
'in' is the link.
(Other definitions for treason that I've seen before include "Disloyalty (to one's country)" , "Lese-majesty" , "Senator (anag)" , "attempting to overthrow the king" , "Offence against state" .)