D-d-dog that's vicious? (8)
I believe the answer is:
cerberus
If this is the answer, I'm afraid I cannot explain it.
Can you help me to learn more?
(Other definitions for cerberus that I've seen before include "Hades gate guardian" , "Three-headed dog guarding entrance to Hades" , "Guardian dog of Hades" , "Three headed guard dog of Hades" , "Three-headed watchdog that guarded Hades" .)