Expert carries a lot of weight now (2,4)
I believe the answer is:
at once
'now' is the definition.
(thesaurus)
'expert carries a lot of weight' is the wordplay.
'expert' becomes 'ace' (ace is a kind of expert).
'carries' indicates putting letters inside.
'a lot of weight' becomes 'ton'.
'ace' enclosing 'ton' is 'AT ONCE'.
(Other definitions for at once that I've seen before include "A no etc (anag)" , "Instantly" , "Immediately, without delay" , "simultaneously" , "This instant, hop to it" .)