Inscrutable one (6)
I believe the answer is:
sphinx
'inscrutable one' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sphinx that I've seen before include "Legendary puzzle setter" , "Inscrutable character" , "Enigmatic person; Egyptian statue" , "Egyptian statue with lion's body and human head" , "Ancient riddler" .)