Senator sacked for disloyalty (7)
I believe the answer is:
treason
'disloyalty' is the definition.
(treason is a kind of disloyalty)
'senator sacked' is the wordplay.
'sacked' indicates an anagram.
'senator' with letters rearranged gives 'TREASON'.
'for' acts as a link.
(Other definitions for treason that I've seen before include "subversion" , "Crime agains one's country" , "Betrayal of the state" , "One rats (anag)" , "LEse-majestE" .)