Fighter happy a riot erupted (9)
I believe the answer is:
gladiator
'fighter' is the definition.
(gladiator is a kind of fighter)
'happy a riot erupted' is the wordplay.
'happy' becomes 'glad' (similar in meaning).
'erupted' is an anagram indicator (I've seen 'erupting' mean this).
'a'+'riot'='ariot'
'ariot' anagrammed gives 'iator'.
'glad'+'iator'='GLADIATOR'
(Other definitions for gladiator that I've seen before include "Colosseum combatant" , "Professional combatant in ancient Rome" , "One scrapped" , "Gold tiara (anag.)" , "Circus performer" .)