Fighter, happy one attending common soldiers (9)
I believe the answer is:
gladiator
'fighter' is the definition.
(gladiator is a kind of fighter)
'happy one attending common soldiers' is the wordplay.
'happy' becomes 'glad' ('glad' can be a synonym of 'happy').
'one' becomes 'i' (Roman numeral).
'attending' becomes 'at' (I've seen this in other clues).
'common soldiers' becomes 'or' (military abbreviation for 'other ranks').
'glad'+'i'+'at'+'or'='GLADIATOR'
(Other definitions for gladiator that I've seen before include "Circus performer" , "Warrior" , "Roman prize-fighter" , "Professional combatant in ancient Rome" , "Roman arena fighter" .)