Concerning a fight (5)
I believe the answer is:
about
'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 "On the topic of" , "in the offing?" , "round" , "Close to" , "Approximately, around" .)