Riot organised by group of three (4)
I believe the answer is:
trio
'group of three' is the definition.
(eg group of three musicians)
'riot organised' is the wordplay.
'organised' indicates anagramming the letters.
'riot' anagrammed gives 'TRIO'.
'by' acts as a link.
(Other definitions for trio that I've seen before include "Group of three (musicians)" , "Threesome" , "Work for three" , "Beethoven's 'Archduke', for example" , "Band of three" .)