Extremely, but not unspeakably? (7)
I believe the answer is:
utterly
'extremely' is the definition.
(thesaurus)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for utterly that I've seen before include "In all respects" , "'All is changed, changed . . .' (Yeats)" , "Completly and without qualification" , "Completely and without qualification" , "Downright" .)