In dissent, I mentioned opinion (9)
I believe the answer is:
sentiment
'opinion' is the definition.
('sentiment' can be a synonym of 'opinion')
'in dissent i mentioned' is the wordplay.
'in' indicates the answer is hidden within the clue.
'SENTIMENT' is hidden within 'dissent i mentioned'.
(Other definitions for sentiment that I've seen before include "Emotion, self-indulgent or insincere perhaps" , "Slush" , "judgement" , "General feeling" , "View, opinion, feeling" .)