Very much so, but not unspeakably (7)
I believe the answer is:
utterly
'very' is the definition.
Both the answer and definition are adverbs. Perhaps there's a link between them I don't understand?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for utterly that I've seen before include "Completly and without qualification" , "Absolutely and completely" , "Completely and totally" , "Downright" , "'All is changed, changed . . .' (Yeats)" .)