In a mysterious way (6)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
eerily
(Other definitions for eerily that I've seen before include "In strange or unearthly way" , "In a strange and frightening way" , "In a strangely frightening manner" , "In a strange and frightening manner" , "In spooky or ghostly manner" .)