Great duel contested under formal rules (9)
I believe the answer is:
regulated
'under formal rules' is the definition.
(regulated by rules)
'great duel contested' is the wordplay.
'contested' indicates anagramming the letters (contested or disputed form of the letters).
'great'+'duel'='greatduel'
'greatduel' is an anagram of 'REGULATED'.
(Other definitions for regulated that I've seen before include "Controlled - adjusted" , "As ordered" , "Adjusted, or put under rule" , "checked up on" .)