Nanny or billy? (4)
I believe the answer is:
goat
This clue is a double definition.
'nanny' is the first definition.
(nanny is a female goat)
'billy?' is the second definition.
(billy is a kind of goat)
(Other definitions for goat that I've seen before include "foolish person" , "lecherous sort" , "Beast (of Capricorn)" , "beast with sharp horns" , "Horned beast" .)