Soho open-air boxing ring (4)
I believe the answer is:
hoop
'ring' is the definition.
(synonyms)
'soho open-air boxing' is the wordplay.
'boxing' indicates the answer is hidden within the clue (the solution is contained or boxed up by the clue).
'HOOP' is hidden within 'soho open air'.
(Other definitions for hoop that I've seen before include "Metal ring for croquet and basketball" , "Ring of wood - arch at croquet" , "Large ring" , "Circle, ring" , "Circlet" .)