A curved structure may end a riot (7)
I believe the answer is:
anarchy
'a riot' is the definition.
(thesaurus)
'a curved structure may end' is the wordplay.
'a' becomes 'an'.
'curved structure' becomes 'arch' (architectural structure).
'end' says to take the final letters.
The final letter of 'may' is 'y'.
'an'+'arch'+'y'='ANARCHY'
(Other definitions for anarchy that I've seen before include "Lawlessness; chaos" , "Utter lawlessness" , "Absence of government" , "General disorder" , "State of civil disorder" .)