Gladiator's milieu (5)
I believe the answer is:
arena
'gladiator's milieu' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for arena that I've seen before include "Venue for boxing, circus and other action events" , "domain" , "Cheap Prom tickets" , "Area in centre of sports stadium" , "Playing field or boxing ring" .)