Show in Nero's boxing champion (6)
I believe the answer is:
winner
'champion' is the definition.
(winner of a competition)
'show in nero's boxing' is the wordplay.
'boxing' indicates a hidden word (the solution is contained or boxed up by the clue).
'WINNER' is hidden in the letters of 'show in nero'.
(Other definitions for winner that I've seen before include "success" , "The champ" , "Conqueror" , "film director" , "Victor" .)