Wild card (5)
'wild card' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for joker that I've seen before include "Jester" , "53rd card in the pack?" , "Playing card" , "trickster" , "Card - wag" .)
(Other definitions for riot that I've seen before include "scream" , "Affray, fighting in the streets" , "Wild disturbance by crowd of people" , "Run amok" , "Violent crowd disturbance" .)