A number participating in duel eventually (6)
I believe the answer is:
eleven
'a number' is the definition.
(I know that eleven is a type of large integer)
'participating in duel eventually' is the wordplay.
'participating in' indicates a hidden word.
'ELEVEN' is hidden within 'duel eventually'.
(Other definitions for eleven that I've seen before include "Prime number" , "Number in a cricket team" , "Time for morning coffee" , "Sporting side" , "Football team; number" .)