Awful matricide is completely opposed (9)
I believe the answer is:
diametric
'completely opposed' is the definition.
(I've seen this in another clue)
'awful matricide' is the wordplay.
'awful' indicates an anagram.
'matricide' anagrammed gives 'DIAMETRIC'.
'is' acts as a link.
(Other definitions for diametric that I've seen before include "Completely opposed" , "Absolutely opposed to as matricide" , "opposite" .)