Least explainable (7)
I believe the answer is:
eeriest
'least explainable' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for eeriest that I've seen before include "beating stranger" , "Strangest" , "Most ghostly" , "Weirdest, I.e., trees are wierd" , "Most weird" .)