Fighter happy a riot broken up (9)
I believe the answer is:
gladiator
'fighter' is the definition.
(gladiator is a kind of fighter)
'happy a riot broken up' is the wordplay.
'happy' becomes 'glad' (similar in meaning).
'broken up' indicates anagramming the letters.
'a'+'riot'='ariot'
'ariot' with letters rearranged gives 'iator'.
'glad'+'iator'='GLADIATOR'
(Other definitions for gladiator that I've seen before include "One scrapped" , "Man appearing in ring" , "Roman arena combatant" , "Circus performer" , "Film" .)