A boxing match is concerning (5)
I believe the answer is:
about
'is concerning' is the definition.
(about can mean concerning or relating to)
'a boxing match' is the wordplay.
'boxing match' becomes 'bout' (bout can mean a boxing match).
'a'+'bout'='ABOUT'
(Other definitions for about that I've seen before include "In conection with, all around" , "Encompassing" , "Concerning; roughly" , "Apropos, concerning" , "Concerned with" .)