Heavyweight filling in expert without delay (2,4)
I believe the answer is:
at once
'without delay' is the definition.
(I've seen this before)
'heavyweight filling in expert' is the wordplay.
'heavyweight' becomes 'ton' (a ton weighs a lot).
'filling in' indicates putting letters inside.
'expert' becomes 'ace' (ace can informally mean very good).
'ton' placed into 'ace' is 'AT ONCE'.
(Other definitions for at once that I've seen before include "Immediately, this instant" , "Right now, this instant" , "Octane (anag)" , "This instant - no delay (2,4)" , "This minute!" .)