Gladiatorial sites (6)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">gladiatorial sites</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for arenas that I've seen before include "Sports venues" , "Areas of endeavours" , "Areas for events, sporting perhaps" , "Sites of gladiatorial combat" , "Level areas in centre of sports stadiums" .)