A boxing contest is concerning (5)
I believe the answer is:
about
'is concerning' is the definition.
(about can mean concerning or relating to)
'a boxing contest' is the wordplay.
'boxing contest' becomes 'bout' (a bout is a boxing match).
'a'+'bout'='ABOUT'
(Other definitions for about that I've seen before include "In the area of or in relation to" , "Moving" , "Approximately, or in vicinity" , "Regarding" , "in action" .)