An age's rising ground (5)
I believe the answer is:
arena
'ground' is the definition.
(I've seen this before)
'an age's rising' is the wordplay.
'age' becomes 'era' (both can mean a long time).
'rising' says the letters should be written in reverse (going up in a down clue).
'an'+'era'='anera'
'anera' in reverse letter order is 'ARENA'.
(Other definitions for arena that I've seen before include "Sporting enclosure" , "Venue for contests" , "Area of combat" , "Ring; stage" , "Place of activity, especially conflict" .)