Fighter delighted a riot's broken out (9)
I believe the answer is:
gladiator
'fighter' is the definition.
(I've seen this before)
'delighted a riot's broken out' is the wordplay.
'delighted' becomes 'glad' (I've seen this before).
'broken out' indicates an anagram.
'a'+'riot'='ariot'
'ariot' anagrammed gives 'iator'.
'glad'+'iator'='GLADIATOR'
(Other definitions for gladiator that I've seen before include "Warrior" , "Roman arena combatant" , "One trained to fight in Roman arenas" , "Circus performer" , "Film" .)