Vicious dog (3)
I believe the answer is:
cur
'vicious dog' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cur that I've seen before include "Inferior dog or one of mixed breed" , "Mongrel - coward" , "A worthless dog or contemptible person" , "Non-pedigree dog" , "Lowly dog" .)