Good circular stadium (6)
I believe the answer is:
ground
'stadium' is the definition.
(eg a football ground)
'good circular' is the wordplay.
'good' becomes 'g' (abbreviation).
'circular' becomes 'round' (synonyms).
'g'+'round'='GROUND'
(Other definitions for ground that I've seen before include "The solid surface of the Earth" , "cause" , "Terrain" , "Minced" , "Arena" .)