Weirdly (6)
I believe the answer is:
eerily
'weirdly' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for eerily that I've seen before include "in a disturbing way" , "In a strange and frightening way" , "In a strange and frightening manner" , "alarmingly" , "In strange or unearthly way" .)