Treachery of the senator (7)
I believe the answer is:
treason
'treachery' is the definition.
(synonyms)
'of the senator' is the wordplay.
'of' indicates an anagram (the answer comes from or 'of' the letters).
'senator' anagrammed gives 'TREASON'.
(Other definitions for treason that I've seen before include "One's art? (anag)" , "Lese-majesty" , "Criminal attempt to overthrow a government" , "Mutiny" , "Disloyalty (to one's country)" .)