Complete and utter (6)
'complete and utter' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sheer that I've seen before include "Very fine or vertical" , "Very steep or diaphanous" , "Very thin and fine" , "Very thin; unmitigated" , "Unqualified" .)
(Other definitions for arrant that I've seen before include "Out-and-out, utter" , "Downright - maybe nonsense" , "Downright - notorious" , "Utter and extreme, maybe nonsense" , "Utter, complete" .)