Stadium reflected a new age (5)
I believe the answer is:
arena
'stadium' is the definition.
(eg a sports arena)
'reflected a new age' is the wordplay.
'reflected' says the letters should be written in reverse.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'age' becomes 'era' (both can mean a long time).
'a'+'n'+'era'='anera'
'anera' back-to-front is 'ARENA'.
(Other definitions for arena that I've seen before include "sports ground" , "Where gladiators fought" , "Sit around here being entertained" , "Ring for contest" , "battlefieId" .)