Most strange and frightening (7)
I believe the answer is:
eeriest
'most strange and frightening' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for eeriest that I've seen before include "Most ghostly" , "Strangest" , "Most strange and frightening" , "beating stranger" , "Weirdest, I.e., trees are wierd" .)