Attack fully-grown kid (4)
I believe the answer is:
goat
I believe this clue is a double definition.
'attack' is the first definition.
(go at)
'fully-grown kid' is the second definition.
(kid can mean a young goat)
(Other definitions for goat that I've seen before include "lecher" , "heavenly sign" , "Butting nanny, perhaps?" , "Domesticated animal supplying milk, meat and wool" , "Billy - nanny" .)