Weight carried by expert without delay (2,4)
I believe the answer is:
at once
'without delay' is the definition.
(I've seen this before)
'weight carried by expert' is the wordplay.
'weight' becomes 'ton' (unit of weight).
'carried by' is an insertion indicator.
'expert' becomes 'ace' (ace can informally mean very good).
'ton' inserted into 'ace' is 'AT ONCE'.
(Other definitions for at once that I've seen before include "simultaneously" , "Immediately, this instant" , "This instant, hop to it" , "Immediately, without delay" , "A no etc (anag)" .)