Expert entertaining a hundred without hesitation (2,4)
I believe the answer is:
at once
'without hesitation' is the definition.
(I've seen this before)
'expert entertaining a hundred' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'entertaining' indicates putting letters inside (inviting in, as for dinner).
'a hundred' becomes 'ton' (I've seen this before).
'ace' placed around 'ton' is 'AT ONCE'.
(Other definitions for at once that I've seen before include "This minute!" , "without pause" , "This instant, hop to it" , "This instant, without delay" , "Right now, this instant" .)