Boxing ring not entirely square, naturally (5)
I believe the answer is:
arena
'boxing ring' is the definition.
(I've seen this before)
'not entirely square naturally' is the wordplay.
'not entirely' says the answer is hidden in the clue.
'ARENA' is hidden in the letters of 'square naturally'.
(Other definitions for arena that I've seen before include "Entertainments venue" , "Contest venue" , "Sporting venue enclosed by seats" , "sports ground" , "Enclosure for sports etc." .)