Behave wildly (3,4)
I believe the answer is:
run riot
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">behave wildly</span>' is the definition.<br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)