Expert bearing weight right away (2,4)
I believe the answer is:
at once
'right away' is the definition.
(I've seen this in another clue)
'expert bearing weight' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'bearing' means one lot of letters goes inside another.
'weight' becomes 'ton' (unit of weight).
'ace' going around 'ton' is 'AT ONCE'.
(Other definitions for at once that I've seen before include "'Without delay (2,4)'" , "This instant, hop to it" , "Immediately, this instant" , "Octane (anag)" , "'Immediately (2,4)'" .)