Displaying guile, Oscar is terribly irreverent (12)
I believe the answer is:
sacrilegious
'terribly irreverent' is the definition.
'displaying guile oscar is' is the wordplay.
'displaying' indicates anagramming the letters (the anagrammed letters display the answer).
'guile'+'oscar'+'is'='guileoscaris'
'guileoscaris' is an anagram of 'SACRILEGIOUS'.
(Other definitions for sacrilegious that I've seen before include "Profane" , "Profaning holy things" .)