Utter in no uncertain sense! (6)
I believe the answer is:
arrant
'utter' is the definition.
(old-fashioned term meaning complete or utter)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for arrant that I've seen before include "Complete (nonsense)" , "Out-and-out, downright - often nonsense" , "Downright - maybe nonsense" , "Extreme or flagrant, perhaps nonsense" , "Rat ran badly and became unmitigated" .)