Terrible riot with rat, a deserter (7)
I believe the answer is:
'a deserter' is the definition.
'terrible riot with rat' is the wordplay.
'terrible' is an anagram indicator.
'with' is a charade indicator (letters next to each other).
'riot' put after 'rat' is 'ratriot'.
'ratriot' is an anagram of 'TRAITOR'.
(Other definitions for traitor that I've seen before include "Unreliable figure" , "Disloyal person" , "One committing treason" , "Betrayer, quisling" , "Double-crosser" .)