Relating to a fight (5)
I believe the answer is:
about
'relating to' is the definition.
(I've seen this in another clue)
'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 "Not exactly - concerning" , "Moving" , "round" , "Regarding" , "in action" .)