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' (I am - referring to the crossword setter).
'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" .)