Small group running riot (4)
I believe the answer is:
trio
'small group' is the definition.
(I've seen this before)
'running riot' is the wordplay.
'running' indicates an anagram.
'riot' with letters rearranged gives 'TRIO'.
(Other definitions for trio that I've seen before include "Musical group" , "Archduke, possibly" , "Beethoven's 'Archduke', for instance" , "Band of three" , "Three together" .)