One round nicks European after gunmen run riot (5,4)
I believe the answer is:
raise hell
'run riot' is the definition.
(both can mean to behave in an unrestrained way)
'one round nicks european after gunmen' is the wordplay.
I cannot really understand how this works, but
'one' could be 'i' (roman numeral) and 'i' is present in the answer.
'european' could be 'e' (abbreviation e.g. EU) and 'e' is found in the answer.
'round' could be 'shell' (both can mean ammunition) and 'shell' is found within the leftover letters.
'gunmen' could be 'ra' and 'ra' is located in the answer.
No letters remain.
This may be the basis of clue (or it may be nonsense).
(Other definitions for raise hell that I've seen before include "cause trouble" , "create commotion" , "Make trouble" , "complain vociferously" , "Complain in the strongest possible terms" .)