Share national boxing stadium (5)
I believe the answer is:
arena
'stadium' is the definition.
(eg a sports arena)
'share national boxing' is the wordplay.
'boxing' says the answer is hidden in the clue (the solution is contained or boxed up by the clue).
'ARENA' is hidden in the letters of 'share national'.
(Other definitions for arena that I've seen before include "Entertainments venue" , "Area surrounded by seating, for sports, etc." , "Sphere of action" , "Playing area" , "Wembley, for example" .)