Three-headed gatekeeper (8)

I believe the answer is:

cerberus

(Other definitions for cerberus that I've seen before include "Underground watchdog" , "Three headed guard dog of Hades" , "Three-headed watchdog that guarded Hades" , "Mythical three-headed watchdog guarding Hades" , "Hades gate guardian" .)

I've seen this clue in The Sun.

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