Opinionated idiot can err horribly (11)
I believe the answer is:
doctrinaire
'opinionated' is the definition.
(following a particular doctrine)
'idiot can err horribly' is the wordplay.
'horribly' is an anagram indicator (a horribly wrong version of the letters).
'idiot'+'can'+'err'='idiotcanerr'
'idiotcanerr' anagrammed gives 'DOCTRINAIRE'.
(Other definitions for doctrinaire that I've seen before include "Dogmatic" , "Impractical theorist" , "Seeking to impose a view" , "Inflexible in applying theory" , "not practical" .)