Expert pockets hundred immediately (2,4)
I believe the answer is:
at once
'immediately' is the definition.
(I've seen this before)
'expert pockets hundred' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'pockets' indicates putting letters inside.
'hundred' becomes 'ton' (I've seen this before).
'ace' enclosing 'ton' is 'AT ONCE'.
(Other definitions for at once that I've seen before include "This instant, hop to it" , "without hesitation" , "This instant - no delay (2,4)" , "Immediately, without delay" , "simultaneously" .)