William's little goat? (5)
I believe the answer is:
billy
'goat?' is the definition.
(billy is a kind 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 final letter removed 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" .)