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