Concerning a contest (5)
I believe the answer is:
about
'concerning' is the definition.
(about can mean concerning or relating to)
'a contest' is the wordplay.
'contest' becomes 'bout' (a bout is a boxing match).
'a'+'bout'='ABOUT'
(Other definitions for about that I've seen before include "Apropos, concerning" , "Regarding" , "round" , "Encompassing" , "In conection with, all around" .)