Completely unacceptable reactionary (8)
I believe the answer is:
outright
'completely' is the definition.
(thesaurus)
'unacceptable reactionary' is the wordplay.
'unacceptable' becomes 'out' (I've seen this in other clues).
'reactionary' becomes 'right' ('right' can be similar in meaning to 'reactionary').
'out'+'right'='OUTRIGHT'
(Other definitions for outright that I've seen before include "Without restriction or reservation" , "Completely and thoroughly" , "Completely - undisputed" , "without qualifications" , "Unqualified - without delay" .)