Vigorously attack bearded mammal (4)
I believe the answer is:
goat
This could be a double definition.
'vigorously attack' is the first definition.
'goat' can be an answer for 'attack' (go at). I'm unsure of the 'vigorously' bit.
'bearded mammal' is the second definition.
'goat' can be an answer for 'mammal' (goat is a kind of mammal). I'm not certain of the 'bearded' bit.
Can you help me to learn more?
(Other definitions for goat that I've seen before include "beast with sharp horns" , "lecher" , "Domesticated animal supplying milk, meat and wool" , "heavenly sign" , "foolish person" .)