Filling snack? I demand a little butter (3)
I believe the answer is:
kid
'butter' is the definition.
(I know that kid is a type of goat)
'filling snack? i demand a little' is the wordplay.
'a little' indicates a hidden word (a little bit of the clue).
'KID' is hidden within 'filling snack i demand'.
(Other definitions for kid that I've seen before include "Young animal or child" , "Soft leather" , "Tease; hoodwink" , "To fool; young animal" , "Sort of glove" .)