Riot set off by group (4)
I believe the answer is:
trio
'group' is the definition.
(a group of three)
'riot set off' is the wordplay.
'set off' indicates anagramming the letters.
'riot' with letters rearranged gives 'TRIO'.
'by' is the link.
(Other definitions for trio that I've seen before include "father, spirit and son" , "Band of three" , "Riot (anag)" , "Work for three" , "Group of Magi?" .)