Duplicity of crooked senator (7)
I believe the answer is:
treason
'duplicity' is the definition.
(thesaurus)
'crooked senator' is the wordplay.
'crooked' indicates anagramming the letters.
'senator' with letters rearranged gives 'TREASON'.
'of' acts as a link.
(Other definitions for treason that I've seen before include "Betrayal (of one's country)" , "attempting to overthrow the king" , "Betraying one's country" , "One's art? (anag)" , "Stare on (anag.)" .)