Go berserk, having to manage tumultuous crowd (3,4)
I believe the answer is:
run riot
'go berserk' is the definition.
(I've seen this before)
'to manage tumultuous crowd' is the wordplay.
'to manage' becomes 'run' (I've seen this before).
'tumultuous crowd' becomes 'riot'.
'run'+'riot'='RUN RIOT'
'having' acts as a link.
(Other definitions for run riot that I've seen before include "Behave in unruly way like mob" , "Lose control" , "Of mob, throw off all restraint" , "go crazy" , "Go on the rampage" .)