Fight sites (6)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fight 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 "Areas in centre of amphitheatres" , "Central areas of, say, sports stadiums" , "rings" , "Areas for events, sports perhaps" , "Areas for events, sporting perhaps" .)