Rabble-rouser finally appearing at a riot, unhappily (8)
I believe the answer is:
agitator
'rabble-rouser' is the definition.
(thesaurus)
'finally appearing at a riot unhappily' is the wordplay.
'finally' indicates one should take the final letters.
'unhappily' is an anagram indicator.
The last letter of 'appearing' is 'g'.
'g'+'at'+'a'+'riot'='gatariot'
'gatariot' is an anagram of 'AGITATOR'.
(Other definitions for agitator that I've seen before include "one causing a stir" , "cause of trouble" , "Political troublemaker" , "Person who stirs up trouble" , "Person stirring up public feeling" .)