Unspeakable (9)
I believe the answer is:
ineffable
'unspeakable' is the definition.
(I know that unspeakable can be written as ineffable)
This is all the clue.
(Other definitions for ineffable that I've seen before include "beyond description" , "Too sacred to be spoken" , "Too great to be put into words" , "Unutterable, too great to be described" , "impossible to describe" .)