The saint wicked? Is the opposite (10)
I believe the answer is:
antithesis
'the opposite' is the definition.
(I've seen this before)
'the saint wicked? is' is the wordplay.
'wicked?' indicates anagramming the letters (I've seen 'wicked' mean this).
'the'+'saint'='thesaint'
'thesaint' with letters rearranged gives 'antithes'.
'antithes'+'is'='ANTITHESIS'
(Other definitions for antithesis that I've seen before include "Opposite; contrasting idea" , "Contrast between two things" , "Reverse" , "I oppose the suggestion" , "Opposition" .)