Unutterable (9)
I believe the answer is:
ineffable
'unutterable' is the definition.
(I know that unutterable can be written as ineffable)
This is the entire clue.
(Other definitions for ineffable that I've seen before include "Inexpressible" , "Too great to be put into words" , "beyond description" , "Indescribable in words" , "Beyond words" .)