Perfect thought comes to pupil (5)
I believe the answer is:
ideal
'perfect' is the definition.
('ideal' can be similar in meaning to 'perfect')
'thought comes to pupil' is the wordplay.
'thought' becomes 'idea' ('idea' can be a synonym of 'thought'**).
'comes to' says to put letters next to each other.
'pupil' becomes 'l'.
'idea'+'l'='IDEAL'
(Other definitions for ideal that I've seen before include "Just perfect, couldn't be better" , "To which one aspires" , "The best conceivable" , "A deli (anag.)" , "Conceptual" .)