Boxing ring (5)
I believe the answer is:
arena
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">boxing ring</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for arena that I've seen before include "where there's competition" , "Auditorium" , "Central area in sports stadium" , "Performance venue" , "Enclosure for sports etc." .)