Expert grasps weight immediately (2,4)
I believe the answer is:
at once
'immediately' is the definition.
(I've seen this before)
'expert grasps weight' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'grasps' indicates putting letters inside.
'weight' becomes 'ton' (unit of weight).
'ace' placed around 'ton' is 'AT ONCE'.
(Other definitions for at once that I've seen before include "This instant, without delay" , "right away" , "without pause" , "This instant - no delay (2,4)" , "simultaneously" .)