Female effortlessly acquires billions, probably (8)
I believe the answer is:
feasibly
'probably' is the definition.
(feasible can mean probable or likely)
'female effortlessly acquires billions' is the wordplay.
'female' becomes 'f' (common abbreviation).
'effortlessly' becomes 'easily' (synonyms).
'acquires' means one lot of letters goes inside another.
'billions' becomes 'b' (abbreviation).
'easily' placed around 'b' is 'easibly'.
'f'+'easibly'='FEASIBLY'
(Other definitions for feasibly that I've seen before include "In a practicable manner" , "Possibly" .)