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