Is the saint upset? Just the opposite! (10)
I believe the answer is:
antithesis
'the opposite' is the definition.
(I've seen this in another clue)
'is the saint upset?' is the wordplay.
'upset?' indicates anagramming the letters (I've seen 'upset' mean this).
'is'+'the'+'saint'='isthesaint'
'isthesaint' is an anagram of 'ANTITHESIS'.
'just' acts as a link.
(Other definitions for antithesis that I've seen before include "I oppose the suggestion" , "Reverse" , "Direct opposite" , "Opposite; contrasting idea" , "opposing view" .)