Heavy weight carried by wizard immediately (2,4)
I believe the answer is:
at once
'immediately' is the definition.
(I've seen this before)
'heavy weight carried by wizard' is the wordplay.
'heavy weight' becomes 'ton' (a ton weighs a lot).
'carried by' means one lot of letters goes inside another.
'wizard' becomes 'ace' (synonyms).
'ton' going into 'ace' is 'AT ONCE'.
(Other definitions for at once that I've seen before include "Instantly" , "Right now, this instant" , "Immediately, this instant" , "Immediately, without delay" , "'Immediately (2,4)'" .)