An illegal dispatch (13)
I believe the answer is:
assassination
'dispatch' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for assassination that I've seen before include "(Eg, political) murder" , "Politically-inspired murder" , "Political killing" , "Merciful, lenient" , "Ordered killing" .)