One hundred taken on by champion simultaneously (2,4)
I believe the answer is:
at once
'simultaneously' is the definition.
(I've seen this in another clue)
'one hundred taken on by champion' is the wordplay.
'one hundred' becomes 'ton' (ton can informally mean a hundred).
'taken on by' indicates putting letters inside (inserted letters get taken on or taken in).
'champion' becomes 'ace' (both can mean a person excelling at something).
'ton' inserted inside 'ace' is 'AT ONCE'.
(Other definitions for at once that I've seen before include "simultaneously" , "'Without delay (2,4)'" , "right away" , "'Immediately (2,4)'" , "from the kick-off" .)