Plumb (7)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
'plumb' is the definition.
(synonyms)
This is all the clue.
(Other definitions for utterly that I've seen before include "Completly and without qualification" , "Completely and without qualification" , "Absolutely and completely" , "'All is changed, changed . . .' (Yeats)" , "Thoroughly, wholly" .)