A fight's concerning (5)
I believe the answer is:
about
'concerning' is the definition.
(about can mean concerning or relating to)
'a fight's' 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 "Concerning; approximately" , "On the topic of" , "In the area of or in relation to" , "nearby" , "in the offing?" .)