Amphitheaters (6)
I believe the answer is:
'amphitheaters' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for arenas that I've seen before include "Structures for open-air events" , "Stadia" , "Structures for open-air sports or entertainments" , "A snare for central parts of amphitheatres" , "Spaces for sports events" .)