Group set off riot (4)
I believe the answer is:
trio
'group' is the definition.
(a group of three)
'set off riot' is the wordplay.
'set off' indicates an anagram.
'riot' is an anagram of 'TRIO'.
(Other definitions for trio that I've seen before include "Beethoven's 'Archduke': famous piece written for this ensemble" , "Group of three (musicians)" , "A group or set of three" , "Small group playing" , "Ensemble musical piece, eg Beethoven's 'Archduke'" .)