Spooky vibe energises heartless monster (8)
I believe the answer is:
eeriness
'monster' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for eeriness that I've seen before include "Ghostly atmosphere" , "Weird quality" , "'Weirdness, scariness (8)'" , "Strange and frightening quality" , "'Ghostliness, scariness (8)'" .)