Sphere of activity (5)
(Other definitions for field that I've seen before include "The ... (John B Keane)" , "Pasture or crop land" , "Area of farmland" , "Flodden?" , "Area for crops/livestock" .)
'sphere of activity' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for arena that I've seen before include "where there's competition" , "Scene of combat" , "Ring for contest" , "Sporting enclosure" , "Wembley, for example" .)