Completely wrong? Correct (8)
I believe the answer is:
outright
'completely' is the definition.
(thesaurus)
'wrong? correct' is the wordplay.
'wrong?' becomes 'out' (eg a calculation might be 'out').
'correct' becomes 'right' ('right' can be a synonym of 'correct').
'out'+'right'='OUTRIGHT'
(Other definitions for outright that I've seen before include "Directly; without reservation" , "Completely - immediately" , "Totally" , "Immediately; openly" , "Total" .)