Group causing riot (4)
I believe the answer is:
trio
'group' is the definition.
(a group of three)
'causing riot?' is the wordplay.
'causing' indicates an anagram.
'riot' anagrammed gives 'TRIO'.
(Other definitions for trio that I've seen before include "Three of a kind" , "Riot (anag)" , "Bee Gees, Supremes or Andrews Sisters, say" , "Three together" , "Archduke, possibly" .)