The group causing a riot (4)
I believe the answer is:
trio
'the group' is the definition.
The answer is an example.
'causing a riot' is the wordplay.
'causing' indicates anagramming the letters.
'riot' anagrammed gives 'TRIO'.
(Other definitions for trio that I've seen before include "Three of a kind" , "(Composition for) three performers" , "Group of Magi?" , "wise men?" , "Three together" .)