William's little goat? (5)
I believe the answer is:
billy
'goat?' is the definition.
(I know that billy is a type of goat)
'william's little' is the wordplay.
'william's' becomes 'Billy's'.
'little' means to remove the last letter (the word is not quite complete).
'billys' with its last letter taken away is 'BILLY'.
(Other definitions for billy that I've seen before include "Camper's pot" , "Cooking can for William" , "Light metal cooking pot used for camping" , "The familiar male goat" , "Goat; camper's kettle" .)