Expert grips weight immediately (2,4)
I believe the answer is:
at once
'immediately' is the definition.
(I've seen this in another clue)
'expert grips weight' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'grips' is an insertion indicator.
'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 minute!" , "Octane (anag)" , "This instant - no delay (2,4)" , "A no etc (anag)" , "right away" .)