Live as good person, being cruel and brutal one? (5)
I believe the answer is:
'cruel and brutal one?' is the definition.
(a cruel person)
'live as good person being' is the wordplay.
'live' becomes 'be' (synonyms).
'good person being' becomes 't' (I can't explain this - if you can you should believe this answer much more).
(Other definitions for beast that I've seen before include "rotter" , "wild creature" , "beef?" , "Traditional poet" , "Monstrous person" .)