Fools adopting a billion animals (9)
I believe the answer is:
wallabies
'animals' is the definition.
The answer is an example.
'fools adopting a billion' is the wordplay.
'fools' becomes 'wallies' (wally can mean a stupid person).
'adopting' means one lot of letters goes inside another (inserted letters are taken on or adopted).
'billion' becomes 'b'.
'a'+'b'='ab'
'wallies' placed around 'ab' is 'WALLABIES'.
(Other definitions for wallabies that I've seen before include "Marsupials" , "Pouched jumpers" , "bounders" .)