Expert bagging century immediately (2,4)
I believe the answer is:
at once
'immediately' is the definition.
(thesaurus)
'expert bagging century' is the wordplay.
'expert' becomes 'ace' (ace is a kind of expert).
'bagging' means one lot of letters goes inside another (I've seen this in other clues).
'century' becomes 'ton' (both can mean one hundred of something).
'ace' going around 'ton' is 'AT ONCE'.
(Other definitions for at once that I've seen before include "This instant, hop to it" , "simultaneously" , "from the kick-off" , "This instant - no delay (2,4)" , "Immediately, this instant" .)