Boxing match, though outside ring (4)
I believe the answer is:
bout
'boxing match' is the definition.
(bout can mean a boxing match)
'though outside ring' is the wordplay.
'though' becomes 'but'.
'outside' indicates putting letters inside (some letters go outside others).
'ring' becomes 'o' (O is the shape of a ring).
'but' enclosing 'o' is 'BOUT'.
(Other definitions for bout that I've seen before include "Short period of intense activity" , "Spell; contest" , "Spell of eg drinking" , "Attack (of illness)" , "Spell - boxing match" .)