Bearded Billy (4)
I believe the answer is:
goat
'billy' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for goat that I've seen before include "Beast (of Capricorn)" , "Capricorn animal" , "heavenly sign" , "lecher" , "Billy - nanny" .)