A fight becomes concerning (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'
'becomes' is the link.
(Other definitions for about that I've seen before include "round" , "Concerning; roughly" , "Approximately, around" , "Circa" , "Encompassing" .)