One of Caesar's last words, the beast! (5)
I believe the answer is:
brute
I believe this clue is a double definition.
'one of caesar's last words' is the first definition.
(as in 'et tu, Brute?')
'the beast' is the second definition.
('brute' can be a synonym of 'beast')
(Other definitions for brute that I've seen before include "'Et tu, - - !' (J.Caesar)" , "Vicious type" , "Savage animal" , "Solely physical" , "Wild animal" .)