Unacceptable to entertain independent thought (6)
I believe the answer is:
notion
'thought' is the definition.
(notion is a kind of thought)
'unacceptable to entertain independent' is the wordplay.
'unacceptable' becomes 'noton' (I've seen this before).
'to entertain' means one lot of letters goes inside another.
'independent' becomes 'i'.
'noton' placed around 'i' is 'NOTION'.
(Other definitions for notion that I've seen before include "A vague and possibly incorrect opinion" , "Caprice" , "Whim or fancy" , "Idea or fancy" , "Idea or whim" .)