Setter's opposed to editor's thought (8)
I believe the answer is:
imagined
'thought' is the definition.
('imagine' can be a synonym of 'think')
'setter's opposed to editor's' is the wordplay.
'setter's' becomes 'im' (referring to the person who set the crossword).
'opposed to' becomes 'agin' (dialect form of against).
'editor' becomes 'ed' (abbreviation).
'im'+'agin'+'ed'='IMAGINED'
(Other definitions for imagined that I've seen before include "thought" , "Conceived of" , "Dreamed" , "Visualised" , "Fancied" .)