Perfect (but not quite fully formed) hypothesis (4)
I believe the answer is:
idea
'hypothesis' is the definition.
(I've seen this in another clue)
'perfect but not quite fully formed' is the wordplay.
'perfect' becomes 'ideal' (similar in meaning).
'but not quite fully formed' means to remove the last letter.
'ideal' with its final letter taken off is 'IDEA'.
(Other definitions for idea that I've seen before include "Purpose" , "intellectual product" , "Theory" , "Thought; concept" , "Inkling" .)