More spooky (6)
I believe the answer is:
eerier
'more spooky' is the definition.
('eery' can be a synonym of 'spooky')
This is all the clue.
(Other definitions for eerier that I've seen before include "Weirder" , "More strangely frightening" , "More mysterious" , "More strange and frightening" , "More unsettling" .)