Rescue little brother from vicious guard dog (8)
I believe the answer is:
cerberus
'guard dog' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for cerberus that I've seen before include "Three-headed guard dog" , "underworld guard" , "Mythical three-headed watchdog guarding Hades" , "Underground watchdog" , "Three-headed watchdog guarding entrance to Hades" .)