Undead monster (6)
I believe the answer is:
zombie
'undead monster' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for zombie that I've seen before include "Corpse said to have been revived by witchcraft" , "Walking corpse" , "Dead man walking" , "Dead body brought back to life" , "Lethargic person" .)