Heavyweight punches champion immediately (2,4)
I believe the answer is:
at once
'immediately' is the definition.
(I've seen this before)
'heavyweight punches champion' is the wordplay.
'heavyweight' becomes 'ton' (a ton weighs a lot).
'punches' is an insertion indicator (some letters punch a hole in others).
'champion' becomes 'ace' (both can mean a person excelling at something).
'ton' placed inside 'ace' is 'AT ONCE'.
(Other definitions for at once that I've seen before include "Immediately, this instant" , "without hesitation" , "simultaneously" , "This instant, without delay" , "Right now, this instant" .)