It is thought to be almost perfect (4)
I believe the answer is:
idea
'it is thought' is the definition.
(synonyms)
'almost perfect' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'perfect' becomes 'ideal' (similar in meaning).
'ideal' with its final letter removed is 'IDEA'.
'to be' acts as a link.
(Other definitions for idea that I've seen before include "Thought, belief" , "Thought, conception" , "Content of cognition" , "Brainwave" , "Sudden inspiration?" .)