Crime that upsets senator (7)
I believe the answer is:
'crime' is the definition.
(I know that treason is a type of crime)
'upsets senator' is the wordplay.
'upsets' is an anagram indicator.
'senator' anagrammed gives 'TREASON'.
'that' acts as a link.
(Other definitions for treason that I've seen before include "Deliberate act of betrayal" , "Serious crime" , "Stare on (anag.)" , "Act of betrayal of a government" , "Disloyalty (to one's country)" .)