Rampant senator's infidelity (7)
I believe the answer is:
treason
'infidelity' is the definition.
(treason is a kind of infidelity)
'rampant senator's' is the wordplay.
'rampant' is an anagram indicator (I've seen this in other clues).
'senator' anagrammed gives 'TREASON'.
(Other definitions for treason that I've seen before include "Serious crime" , "Senator (anag)" , "Betrayal (of one's country)" , "subversion" , "Deliberate act of betrayal" .)