Thought to be perfect but coming up short? (4)
I believe the answer is:
idea
'thought' is the definition.
(idea is a kind of thought)
'perfect but coming up short?' is the wordplay.
'perfect' becomes 'ideal' (similar in meaning).
'but coming up short?' means to remove the last letter.
'ideal' with its final letter taken away is 'IDEA'.
'to be' acts as a link.
(Other definitions for idea that I've seen before include "Clue" , "Product of the mind" , "Fancy" , "Content of cognition" , "Sudden inspiration?" .)