Riot caused by a group (4)
I believe the answer is:
trio
'a group' is the definition.
The answer is an example.
'riot caused' is the wordplay.
'caused' is an anagram indicator (I've seen 'causes' mean this).
'riot' anagrammed gives 'TRIO'.
'by' is the link.
(Other definitions for trio that I've seen before include "Musical group" , "Piece for three players" , "Work for three" , "Riot (anag)" , "A group or set of three" .)