Almost perfect belief (4)
I believe the answer is:
idea
'belief' is the definition.
(I know that idea is a type of opinion)
'almost perfect' is the wordplay.
'almost' means to remove the last letter (almost the whole word is kept).
'perfect' becomes 'ideal' (similar in meaning).
'ideal' with its final letter taken off is 'IDEA'.
(Other definitions for idea that I've seen before include "Creative spark" , "Notion, concept" , "Aim" , "Purpose" , "Theory" .)