Behave wildly (3,4)
I believe the answer is:
run riot
'behave wildly' is the definition.
This is the entire clue.
(Other definitions for run riot that I've seen before include "Behave in unruly way like mob" , "Behave unrestrainedly" , "Spread uncontrollably" , "behave in unrestrained way" , "Of mob, throw off all restraint" .)